The direction and position given to the centre line of the railway track on the ground is called the track alignment. The horizontal alignment includes the straight path, its width, deviations in width and horizontal curves. The vertical alignment of a railway track includes changes in gradients and
vertical curves.

A new railway track should be aligned very carefully as improper alignment would result either in capital loss in initial cost of construction or recurring loss in maintenance and vehicle operation cost, or both. Once the track has been aligned and constructed, it is difficult to change the alignment due to increase in cost of adjoining land and construction of costly structures over railway track and at specific location by the side of railway line.

Basic Requirements of Good Track Alignment

In case of railways, the profit is not the only objective but comfort to the passengers must be given equal importance. Though the income from transportation of goods is much more than that from passenger traffic but the safety, comfort and convenience of passengers are always kept in view. An ideal alignment should fulfil the following requirements –

1. Purpose of Track

The alignment of the track should be done keeping in view the basic purpose or purposes, it is going to serve. In general, the railways serve the following purposes –

(i) Transportation Services

Railways carry the passenger traffic, and goods traffic, local as well as through.

(ii) Political and Strategic

Sometimes it becomes essential to construct a railway line to connect two points (either with the neighbouring countries or within the country) for defence purposes so that in case of emergency, armies within the country can be transferred from one place to another.

(iii) Linking of Centres

When a railway line is constructed basically to connect important places for providing transport services, it may be linking of two trade centres. In such cases, a new railway line is proposed to connect two trade centres.

(iv) To Open up New Tracks

It may be necessary to align new tracks for the land, whose resources are not yet tapped.

(v) Shortening Existing Track

When an existing track is zig-zag or uneconomical, a new track may be aligned to shorten the existing track which would prove to be economical.

2. Feasibility

For aligning a railway line, it is necessary to carryout feasibility study so that the proposed track alignment melts all technical requirements and also fits in well with the general planning of the country.

3. Economy

The track alignment will be economical when following factors are given due consideration –

1. All other things being equal, the shortest and the most direct route between two joining points is the cheapest. Though, there may be several practical considerations which would cause a deviation from the shortest route.
2. The initial construction cost should be minimum. This can be achieved by avoiding loose earth slopes, rock-cuttings, drainage crossings by aligning the track on watersheds, etc., if feasible.
3. Maintenance cost should be minimum. This is more important than the construction cost, being recurring in nature. Maintenance cost can be reduced by avoiding deep cutting, very high banks, long viaducts, tunnels and heavy gradients which cause heavy wear on rails and rolling stock.
4. The operating expenses or transportation cost should be minimum. The transportation cost will be minimum, when the haulage of goods is maximum for the given power of locomotive and type of traction. This can be achieved by providing easy gradients, avoiding unnecessary rises and adopting shortest-direct-route.

4. Safety

The track should be so aligned that it gives safety to traffic operations or in other words the passengers and goods traffic can be transported without any chance of accident or derailment.
To achieve this, the track should be properly designed from the viewpoint of curves, gradients and at the same time stability of natural slopes, embankment and cut-slopes. Foundations of embankments should be properly maintained.

5. Aesthetic Aspects

This aesthetic aspect of a railway line should be kept in view for comfortable and pleasant railway journey. This is achieved by avoiding the view of borrow pits, use of transition curves, etc., or keeping the track through beautiful natural surroundings.

Factors in selection of good track alignment

Though the direct shortest route is the most economical but is rarely possible due to various practical difficulties such as intermediate obstructions, steep gradients within the shortest route, construction and maintenance problems etc. Moreover, it may be necessary to deviate from the shortest route to connect obligatory points. (i.e. traffic generating points or places of importance).
An alignment, which is economical in the initial cost, may not prove most economical in maintenance or vehicle operation cost or both. It is also possible that shortest route may be costlier than other routes, when different alternatives are tried from construction point of view. Thus, it may be concluded that all the requirements cannot be fully justified simultaneously.

For satisfying most of the requirements, the following factors in selection of good alignment, require due consideration –

1. Obligatory or controlling points
2. Position, Amount and Type of Traffic
3. Gauge-Selection
4. Geometric Standards
5. Topography of the country
6. Economic Considerations
7. Other Considerations

1. Obligatory or Controlling Points

These are the points which govern the alignment of a railway track. These points can be mainly classified into two categories –

(i) Points through which a track must pass

The points through which a track must pass after deviating from shortest route are the following –

(a) Important Towns and Cities

It is desirable that a track alignment should pass from places of social, commercial, political and defence importance. This would further depend upon importance of the town, potential volume of traffic to justify revenues, physical features of the country and amount of deviation from shortest path to connect these points.

(b) Major-Bridges or River Crossings

The construction of major bridges over large rivers is a costly affair so the bridge sites are very important obligatory points for selection of alignment.

(c) Hill-passes or Saddles

To avoid unnecessary requirements of deep cuttings or high banks, or tunnels or viaducts, the existing passes or sadles
should be connected, whenever crossing the hills.

(d) Site for Tunnels

Though passing of a track through tunnel should be avoided as far as possible but in case of high and thin ridges when a tunnel can be an economical preposition by avoiding deep cuttings, the track should be aligned through such tunnel sites.

(ii) Points through which a track should not pass

The points through which a track alignment should not pass are the
following –
(a) Acquisition of costly level which will result in heavy compensation should be avoided.
(b) Religious places like temple, church, mosque or tomb should be avoided as they are protected by law.
(c) Areas liable to flooding (i.e., water logged areas), marshy areas, cutting in snowfall areas or such unsuitable areas of alignment should be avoided.

2. Traffic – Its Position, Nature and Amount

The position of traffic sources furnishes control points for general location of the alignment. The nature of traffic (passenger or goods traffic) and potential volume of traffic govern the type of construction to justify the revenues. So it is essential to estimate the growth of traffic, its position, nature and amount before aligning the track. Following are the general considerations about the traffic –

• Traffic varies as the square of the population. Therefore, the route, which has the greatest population, is the best.
• Freight earnings are much more than passenger earnings.
• Whenever a new line is aligned, all the traffic from an influence area upto a width of 15 km on either side would go to the railway line in the beginning and after few years this width increases to 25 km.

So while conducting traffic surveys all the prospects of traffic-growth and impact within influence area should be studied before aligning any track.

3. Gauge Selection

Gauge has already been discussed. In brief, it can be said that though the increase in width of gauge increases initial cost but it also increases load carrying capacity and the speed of the trains.

4. Geometric Standards

An engineer should design the location of a new line considering the following Geometric elements, which would give economical combination of construction and operation costs.

• It should be kept in view that the performance of locomotives as also efficiency depends upon varying conditions of gradients, speed, loading and method of traction used.
• Ruling gradient and minimum permissible radius of the curve must be considered while making the alignment of the track as it limits the weight and length of trains and thus affects the cost of operation.
• Lesser the radius of curve more would be the operating expenses due to curve resistance. So curves should be of maximum possible radii from economic point of view.
• Wherever reverse curves are necessary to use, long lengths of straight portion (minimum 36 m length) should be provided in between two reverse curves.
• The use of curves should be particularly avoided on approaches and at stations as well as on bridges.
• All the large bridges should be crossed by the alignment at right angles and by use of atleast 30 m straight portion on either side of the bridge.
• The effect of rise and fall of ground should be considered, as they affect the cost of alignment and operative expenses. Ineffective rise should be avoided as far as possible.
• The effect of distance on operating expenses should be considered in the alignment selection. The railway passenger rates and freight charges are fixed according to distance hauled.
• The use of steep gradients increases the operating costs considerably.

5. Topography of the Country

If topography of country is such that the use of heavy gradients is unavoidable then in such cases the alignment of the track is made by the special ways to reduce the rate and cost of high-gradients. For alignment, the topography can be classified under the following heads –

• Valley alignment
• Cross-country-alignment
• Mountain alignment

(i) Valley Alignment

This alignment as such creates no problems. If two control points lie in the same valley, the engineers should choose a straight line between terminal points with a uniform rate of gradient. The ends having steeper gradients can be further negotiated by some form of development.

(ii) Cross Country Alignment

This alignment crosses the water-sheds of two or more streams of different sizes. Unlike the valley alignment, the uniform grade is not possible because the rail levels are so high that there are no appreciable sags or summits between bridges. So the routes in cross-country have sags and summits in succession. The lowest saddles or short tunnel lines may work as the controlling or obligatory points.

(iii) Mountain Alignment

This type of alignment is best obtained by development accompanied by tunneling. For this development, the length of alignment is first increased between the two extreme ends of elevation so as to limit the gradient maximum upto ruling gradient. This development is accomplished by the following special ways –

(a) By Zig-Zag Development – In this, at a favorable point the line makes a half circle loop and returns upon itself in order to gain height at the expense of losing distance. This line is in succession of such loops up the side-valleys or round the hill side alternatively with the principle of maintaining convenient side-slope which is at right-angles to the direction of alignment.

(b) By Switch-Back Development – In case of very steep-side slopes a considerable gain is accomplished by switch-backs. In Fig, A and B are the switches and C and D are buffer stops. The vertical curve is provided to bring the train to a stop slowly and gradually without the use of brakes. So the train stops at switch point and hence the switches preferably should be located in a station yard.

(c) By Spirals or Complete Loops Development – A spiral is a doubling back of the line upon itself, round the sides of a valley or basin of a hill, so that it crosses and returns under-itself at a lower elevation.

Spirals are of two types –

• A bridge-spiral swings around the slopes of a valley where a bridge or viaduct can span the valley at considerable height. So bridge-spiral can be used for a narrow valley.

• A tunnel spiral curves round the slopes of a hill. This gives more favourable gradients than in descending with the slope of the valley on one side only as in case of zig-zag development.

6. Economic Considerations

Keeping in view the above factors, the alignment should be economical also. For this purpose, different alternatives of alignment keeping in view the basic purposes should be analysed from cost point of view, The best alignment would be one, which gives the maximum annual return (r) given by the formula –

This shows ‘r’ should be maximum which depends upon the proper selection of the route so as to include the advantages of traffic generating centres. The other way of increasing the annual returns is sound track construction which reduces the annual running expenses. So a suitable balance between the construction cost and operating expenses should be strack. The biggest item of expenditure is the maintenance of the track, so a reasonable economy in selection of materials and construction should be made.

7. Other Considerations

(i) While aligning the track Geological formation should be studied for the following reasons :

• The alignment should be made on good soil. Otherwise in bad or marshy soil, the drainage problems will increase the cost.
• Alignment should be so laid that it is free of slips or slides.
• The alignment should not be on rocky soil. As it results in costly excavation and also makes the possibility of embankment to slip.

(ii) The effect of flood and climate on the track alignment is important due to following reasons :

• Waterlogged areas should be avoided.
• Cutting in snowfall areas should be avoided as it results in trouble of clearing the track all the time.
• In cold region, the track hould be aligned on that side of hill which is not affected by winds. A sun facing side is desirable.
• The track should cross the road wherever level crossing is there at right angles to avoid accidents.
• The station sites on the track should be selected outside the town or city. It should be on a level stretch of ground.
• The track should pass, if possible, in the proximity of quarries or villages. Because the stones are easily available from quarries for construction and maintenance, and labour is cheaply available from the villages.
• For aligning the track, political considerations should be borne in mind. Such as foreign countries should be avoided for aligning the track. Otherwise in peace time they will charge certain duties, create complications, and in war-time, the link would remain cut off. So the railway lines should be very much away from the boundaries of the country to provide safety during war.

Rack Railways

In case of gradients steeper than 1 in 30, an arrangement, termed as “rack” or “rack and pinion”, is adopted. To haul the trains along such rising gradient, and hence this system of railways is termed as; ‘Rack Railways’ or ‘Rack and Pinion Railways’. In this system of railways, three rails system consisting of two ordinary rails and a third toothed rail generally placed in the centre of a track; is adopted. The locomotive is provided with a toothed pinion wheel; the teeth of which fit into the teeth of the central toothed rail. There are various systems of these rack railways, such as Abt system, Fell system, Marsh system and Riggenbach System.

The use of such rack railways is warranted on steeper gradients. Because it becomes impossible for wheels to stick to rails; and there is every possibility of the train going down the hill slope even though; it is being pushed up the rising gradient. It is quite surprising and interesting to note that a gradient, as steep as 1 in 2 has been adopted with the help of this system. In India, a distance of about 20 km from the ghat at Kullar to Coonoor on Nilgiri railway, is constructed at a uniform slope of 1 in 12 and is being operated on the Abt system. On Nilgiri Rack Railways, even though 75% of the whole line is laid on the curves but no compensation is allowed for curvature.

In rack railways, sleeper, either of hard wood or preferably of steel, are attached with rack chains. To reduce friction at the curves, a jet of hot water is squirted under the wheels.

Ballast is the granular material usually broken stone or brick, shingle or kankar, gravel or sand. It provides a suitable foundation for the sleepers. It also holds the sleepers in their correct level and position. The lateral stability of a track depends on it.

Functions of ballast

Ballast performs the following functions –

• Transfers the load from the sleepers to the subgrade and then distributes it uniformly over a larger area of the formation.
• Holds the sleepers in position and prevents the lateral and longitudinal movement due to load transfer.
• Imparts some degree of elasticity to the track.
• Provides easy means of maintaining the correct levels of two lines of a track.
• Corrects track alignment.
• Provides good drained foundation immediately below the sleepers and help to protect the top surface of the formation.

Requirements / Characteristics of the good ballast

To perform above mentioned functions, the ballast should have following characteristics –

• Able to withstand hand picking without disintegrating.
• Resist crushing under dynamic loads.
• Therefore, it should not be able to make the track dusty or muddy.
• Must have good drainage with minimum soakage.
• Voids should be large enough to prevent capillary action.
• Offer resistance to abrasion and weathering.
• Retain its position laterally and longitudinally under conditions of traffic, specially on curves.
• Should not produce any chemical action with rail and metal sleepers.
• Size of stone should be 5 cm for wooden sleepers, 4 cm for metal sleepers, 2.5 cm for turn outs and cross-overs.
• Easily available so that it reduces the cost of supply.

Types of Ballast

Different materials used as ballast in India are broken stone, gravel, sand, ashes, cinders. And sometimes soft aggregates like moorum, kankar, overburnt and broken brickbats, blast furnace slag and selected earth.

1. Broken Stone

This is the best material for ballast and almost all important tracks are provided with stone ballast. It satisfies all the specifications and requirements of good ballast as mentioned above. The best stone is nonporous, hard and angular, which does not flake when broken. Igneous rocks such as hard trap, quartzite, granite make excellent ballast. These are used in large quantities for high speed tracks on India.

2. Gravel / Shingle / River Pebbles

Gravel comes next in rank for its suitability for use a ballast; thus used in large quantities in many countries. This is obtained either from river beds or from gravel pits. The smooth pebbles are broken, otherwise they are liable to displace the sleeper due to smoothness.

The process of ramming the ballasts underneath the sleeper is known as Packing. The ballasts above this layer which surrounds the sleeper, is loose filled and termed as Boxing. The loose ballasts between the two adjacent sleepers is Ballast Crib.

3. Ashes / Cinders

This material is available in large quantities on railways from coal being used in locomotives. It has excellent drainage properties as it is very porous. It is cheap and largely used in sidings. But can not be used for main lines as it is very soft and reduce the powder on track. It is excellent for station yards and for footpaths. It doesn’t retain water and is not slippery.

Greater drawback is, it’s corrosive in nature. Therefore it corrodes steel sleepers and foot of the rails. In emergencies, due to its easy availablity, can be used for repairing formation or packing tracks.

4. Sand

Its reasonably good material as it is cheap and provides good drainage. Sand also produce a silent track and is good for packing pot sleepers.

Greater drawback of the sand is its blowing effect due to vibration. Sand easily gets on track, thus causes heavy wear. Therefore the maintenance of track is difficult. Sometimes sand layer is covered with stone or brick, to prevent it from blowing. Mostly coarse sand is preferred to fine sand.

5. Moorum

It is the soft aggregate and is the result of decomposition of laterite. It is red or sometimes yellow in colour. The best moorum is that which contains large quantities of small laterite stone.

It is recommended for sidings and main track. Specially when they are new laid and the embankments are not sufficiently consolidated. When moorum is finally laid on the track, it forms a soling or blanket under the stone.

6. Kankar

It is lime agglomerate which is common in certain clayey soils and is dug out of the ground. Where stone is not easily available, it is used as road metal and as ballast for railway tracks. It is soft in nature and reduces to powder under loads. It is used for M.G. and N.G. tracks with light traffic and where a better type of the ballast is not available.

7. Brick Ballast

Where no stone or substitute is available for use as ballast, overbunt bricks are broken into small sizes and used. It powders easily and produces a dusty track. Rails of tracks laid on brick ballasts many a time get corrugated. Brick ballasts, however, is fairly good for drainage.

8. Blast furnace slag

Which is a by-product in the manufacture of pig iron forms a suitable ballasts material. It should, however, be hard, of high density and free from gas holes. Slag, suitable for use as ballasts, is obtained by pouring molten slag collected at the blast furnace into shallow pits of thin layers, allowing it to cool, and then by digging, crushing and screening.

9. Selected Earth

For sidings, earth, if of suitable quality, is sometimes used as ballasts. It is also sometimes used on new formation as a temporary measure. Indurated (i.e. hardened) clay and decomposed rock are suitable materials.

Size and Section of ballast

The size of the ballasts used varies from 1.9 cm to 5.1 cm gauge. Stones of larger sizes are not desirable and the maximum size as 5.1 cm is preferable, as interlocking of stones of this size is better than that of stone of larger sizes. The best ballast is that which contains stones varying in size from 1.9 cm to 5.l cm with reasonable proportion of intermediate sizes.

The exact size of the ballast depends upon the type of sleeper used and location of the track as below –

• Ballast size for wooden sleepers tracks = 5.1 cm. (gauge)
• Ballast size for steel sleepers tracks = 3.8 cm. (gauge)
• Ballast size for under switches and crossings = 2.54 cm. (gauge)

The section of ballast layer consists of depth of ballasts under the sleepers and the width of the ballasts layer.

The depth of the it under the sleepers is an important factor in the load bearing capacity and uniformity of distribution of load. The more is the depth of ballast more will be the load-bearing capacity. In America, a depth of it equivalent to the sleeper spacing is recommended, because of heavier loads and the closer spacing of sleepers being used in that country. In India, this recommendation will give unnecessarily thicker layer of ballast due to large spacing of sleepers, being used.

The width of the ballast layer is also important as the lateral strength of track depends partly upon the quantity of ballasts used at the ends of the sleepers. The lateral strength increases with increase in width of the ballast layer but there is a limit beyond which no useful purpose is served by widening. This width limit is at 38 cm. to 43 cm. from the end of the sleeper as computed. The Indian standards have recommended dimensions for (i) width of ballast section at the level of the foot of the rail and (ii) for depth of ballast below the sleepers.

Minimum Depth of Ballast Section

Although the lines of equal pressure in ballast through wheel loads are in the shape of a ‘bulb’ yet for simplicity purpose, the load dispersion can be assumed at 45° to the vertical. For uniform distribution of load on the formation, the depth of ballast should be such that the dispersion lines do not overlap each other. From simple geometry of the following figure, the depth of ballast can be calculated as below –

Sleeper spacing (S) = width (w) + 2 x Depth of ballast (Db)
or S = w+ 2 x Db
Minimum Depth of ballast = Db = (S-w)/2

For example, if wooden sleepers are used in track laying with sleeper density as (n +7), the sleeper spacing is 65 cm and width of sleeper is 25 cm.

Then, the minimum depth of ballast from above formula works out to be 20 cm which is minimum depth of ballast generally prescribed on Indian Railways.

Quantity of ballast

The quantity of stone ballasts required per metre tangent length is 1.036 m for B.G., 0.71 m for M.G. and 0.53 m for N.G. of track with standard ballast cushion and also timber sleepers. However, slight variations in quantities occur with the type of sleeper used. Wooden and concrete through sleepers because of their bulk, require slightly less ballasts than pot or trough sleepers.

For curves with super-elevation, the quantity of ballasts is slightly more, because as per the Indian standards, recommended depth at level is provided under the inner edge of the sleeper. Moreover, wider shoulders (about 15 cm or more) than specified above are provided on outside curves to counteract the increased lateral thrust.

Renewal of Ballast

Due to prolonged and repeated movement of wheel loads over the railway track, the ballasts material gets crushed to smaller sizes. The fine crushed particles get filled in the voids and form an impervious layer, preventing the rapid flow (i.e. drainage) of the rain water. It thus creates drainage problem.

Quantity of ballast is also reduced due to –

(a) Blowing away of the ballast by the movement of trains.
(b) Penetration of the ballast in the formation, and
(c) Loss of ballast due to rain water and wind etc.

To make up the loss, the ballast is renewed from time to time. In the process of renewal, the crushed fine material is removed by screening, through the screens as shown in following figure and new material of the desired size is added.

Sleepers are members generally laid transverse to the rails. On sleepers, the rails are supported and fixed, to transfer the loads. The load transfers from rails to sleeper, sleeper to ballast, ballast to subgrade below.

Functions of sleepers

1. hold the rails to correct gauge.
2. hold the rails in proper level or transverse tilt.
3. act an elastic medium in between the ballast and rails to absorb vibrations.
4. distribute the load from rails to ballast (or girders in case of bridges).
5. support the rails at proper level in straight tracks and at proper super elevation on curves.
6. also add to the longitudinal and lateral stability of the permanent track on the whole.
7. also provide means to rectify track geometry during service life.

Requirements of sleeper

1. Sleeper to be used should be economical i.e. they should have minimum possible initial and maintenance cost.
2. Fittings of the sleeper should be such that they can be easily adjusted during maintenance operations.
3. Weight of sleeper should not be too heavy or excessive light i.e. ease for handling.
4. Designs of sleeper should be such that the gauge, alignment of track and levels of the rails can be easily adjusted and maintained.
5. Bearing area of sleeper below the rails seat and over the ballast should be enough to resist the crushing due to rail seat; and crushing of the ballast underneath the sleeper.
6. Sleeper design and spacing should be such as to facilitate easy removal and replacement of ballast.
7. Design of the sleeper should be such that they are not pushed out easily due to moving trains especially with steel sleeper with rounded ends.
8. Insulation of rails should be possible for track circuiting, if required, through sleepers.

Classification of sleepers

Sleepers can be classified according to the materials used in their construction, in the following categories –

1. Wooden sleeper
2. Metal sleeper (i) Cast iron (ii) Steel
3. Concrete sleeper (i) Reinforcement concrete (ii) Prestressed concrete

Different types of sleepers are manufactured, are described in the following steps –

1. Wooden or Timber sleepers

Wooden sleepers are regarded to be best. These are ideal sleepers. The life of timber sleepers depends upon their ability to resist

• Wear
• Decay
• Attack by vermin i.e. white ants
• Quality of timber used

Advantages

• Timber is easily available.
• Fittings for wooden sleeper are few and simple in design.
• Wooden sleeper is easy to lay, relay, pack, lift and maintain.
• Wooden sleeper is suitable for all types of ballast.
• Overall more economical.

Disadvantages

• The sleepers are subjected to wear, decay, attack by white ants, spike killing, wrapping, cracking, end spitting, rail cutting etc.
• It is difficult to maintain the gauge in case of wooden sleepers.
• Track is easily disturbed i.e. alignment maintenance is difficult.
• Service life is minimum (12 to 15 years) aa compared to other types of sleepers.
• Maintenance cost is highest.

Types of Timber for sleepers

• Hard wood such as sal and teak
• Soft wood such as chir and deodar

2. Metal sleepers

Due to growing scarcity of wooden sleeper, their high maintenance cost and short life; metal sleepers are now being adopted in India.

Metal sleeper are either of steel or cast iron. Cast iron is largely used because it is less prone to corrosion.

Advantages

• Metal sleepers are uniform in strength and durability.
• In metal sleeper, the performance of fittings is better and hence lesser creep occurs.
• Metal sleepers are economical, as life is longer and maintenance is easier.
• Gauge can be easily adjusted.
• For metal sleeper, frequent renewal is not required.
• They have good scrap value and not susceptible to fire hazards.

Disadvantages

• More ballast is required.
• Greater number of fittings are required, which is difficult to maintain and inspect.
• These are liable to rusting/corrosion.
• Metal being good conductor of electricity interferes with tracking circuit.
• These are unsuitable for bridges, level crossings and also points and crossings.
• Therefore these are suitable only for stone ballast.

Types of cast iron sleeper

• Pot or bowl sleeper
• Plate sleeper
• Box sleeper
• C.S.T.-9 sleeper (combination of plate and box type)
• Rail free duplex sleeper

Types of steel sleeper

• Lugs or Jaws pressed out of metal
• With loose lugs or Jaws
• Clip bolt type
• Saddle or spring type

3. Concrete sleepers

These sleeper were ended due to chronic shortage of good wooden sleeper and need for better design and economy of sleeper on sustainable basis.

Advantages

• These sleeper are free from natural decay and attacks by vermin, insects etc.
• Maximum life time i.e. about 40 to 60 years.
• Not affected by moisture, chemical action of ballast, cinder and sub-soil salt.
• There is no difficulty in the track circuiting also, required for electrifying the track.
• Greater stability of track and thus better resistance against temperature variation.
• The sleeper have higher elastic modulus and hence withstand stresses induced by fast and heavy traffic.
• Concrete sleeper in the elastic fastenings offer an ideal track in every respect such as gauge, cross-level, alignment.

Disadvantages

• Weight of sleeper is as high as 2 to 3 times of wooden sleeper, requiring mechanical appliances for handling.
• These sleeper require pads and plugs for spikes.
• They damage the bottom edge during the packing.
• Scrap value is almost nil.
• Damages to the concrete sleeper is very heavy in case of derailment.

On the other hand, main disadvantages of Prestressed concrete sleeper are –

• Heavily damaged in case of derailments.
• The bed of the ballast is specially prepared.
• These are uneconomical.
• More rigid in nature.
• Design is complicated.

Types of concrete sleepers

These sleeper are mainly of two types –

• Reinforced concrete sleeper
• Prestressed concrete sleeper

Further concrete sleeper is of two types –

• Through type
• Composite or block and tie type

Comparison of all types of sleepers

Sleeper density or spacing of sleeper

The space between two adjacent sleepers determines the effective span of the rail over the sleepers. The spacing of sleepers, therefore, in a track depends on axle load and lateral thrust. Axle load which the track is expected to carry and thus lateral thrust of locomotives to which it is subjected.

Therefore, Number of sleepers in a track is indicated by the number per rail length. However the number of sleepers can’t be increased indefinitely as certain minimum space between sleepers is required for packing of ballast.

In India, this minimum packing distance is kept 30.5 to 35.5 cm except at joints. The number of sleepers per rail varies from M+4 to M+7 for main tracks.

In America, this distance is reduced to 25 cm. The number of sleepers per rail varies from M+9 to M+11 for tracks.

Where, M = length of rail in meters

In Britain, N+4 sleepers are used.

Where, N = length of rail in yards

Therefore, sleeper density is the number of sleepers per rail length and it is specified as M+x or N+x.

Factors governing the sleeper density

• Axle load and speed
• Type and section of the rails
• Type of ballast and ballast cushion
• Strength and type of sleeper i.e. bearing area of particular sleeper on the ballast
• Nature of foundation

Spacing of sleeper is always not uniform. Thus three or four sleepers on either side of the joints are kept down close together. The spacing between joint sleeper is 30 to 45 cm, whereas intermediate it is 75 to 90 cm or as per specifications.