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
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.
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.
The track alignment will be economical when following factors are given due consideration –
- 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.
- 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.
- 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.
- 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.
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 –
- Obligatory or controlling points
- Position, Amount and Type of Traffic
- Geometric Standards
- Topography of the country
- Economic Considerations
- 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
(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
- 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.
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.