Geometric Design

Geometric design of railway track Unit 1

Why design?? Most of train derailments are due to the following reasons:  Track defects (civil Engineer )  Vehicular defects Operational defects Load and speed ( safe and economical)

Straight track derail due : Defective cross levels Defective alignment Defective gauge Low joints Curved track derail due to : Improper super elevation Improper radius of curve Improper speed Unequal distribution of loads on two rails

The derailment over turn outs : Gaping points Lifting of toe of switch due to inadequate fittings. Improper assembly of crossing. Excessive wear in switches. Tight gauge and defective gauge ( nose of crossing ).

The following aspects to be studied : Gradients and grade compensation Speed of the train Radius or degree of the curve  Cant or super elevation Curves Widening of gauges on curves

Gradients and grade compensation Gradients : • Any departure of the track from level is known as grade. • The gradient might be upward or downward. Reasons : • To provide uniform rate of rise or fall as far as possible. • To reach the various stations located at different elevations • To reduce the cost of earth work.

Types of gradients : • Ruling gradient Plain terrain 1 in 150 to 1 in 200 ; hilly terrain -1 in 100 to 1 in 150 • Momentum gradient • Pusher or helper gradient • Gradients at station yards 1 in 400 (max) ; 1 in 1000 (min)

Grade Compensation : In order to avoid resistances beyond the allowable limits , the gradients are reduced on curves and this reduction in gradients is called grade compensation. 0.04% -B.G, 0.03% -M.G, 0.02% -N.G

Speed of the train Speed of the train depends on : • Strength of the track • Power of the locomotive The dynamic effect of speed is obtained if its speed is more than 120 k.m.p.h due to the following reasons :  Various parasitic motions such as pitching , rolling  Resonance between the frequency of application of load and elastic oscillation of the track structure as a whole or its components.

 Inertia or springing action of the track in,form and behind the wheels.  Effect of unbalanced weights.  Effect of unspring masses.  Suspension characteristics of the locomotive and carriages. Safe speed on curves : Safe speed to negotiate depends on :  The gauge of the track  The radius of the curve  The distance at which the resultant of the weight of the vehicle and its centrifugal force acts from the centre of the track  Amount of super elevation provided.  The presence or absence of transition curves at the ends of circular curve

Old practice – use of martins formula New revised formula is used nowadays to find the safe speed of the train. The max cant deficiency provided : B.G – 75mm M.G -50mm N.G – 40 mm

Radius or degree of the curve D =1720/R Following are the main effects or objections of providing curves on a railway track :  The working of trains is affected as there is restriction in speed, prevention of heavy locomotive and limitation of length of train.  There are possibilities of derailment or accidents.  There is no equal distribution of load on the curves ( discomfort in riding).  To provide extra strength on tracks along the curves.

Smallest radius and largest degree are restricted on the basics of two factors : • wheel base of vehicle • Sharpness of the curve in India , curves on through tracks , are limited to the following max radii : B.G – 10 degree( R=175m) M.G – 16 degree(R=109m) N.G – 40 degree(R=44m)

Super-elevation Objective :  To introduce centripetal force for counteracting the effect of centrifugal force, this will result in the faster movement of trains on the curves. This will also prevent derailment and reduce the side wear and creep of rails.  To provide equal distribution of wheel loads on two rails so that there is no tendency of track to move out of position due to more load on outer rail. This reduces the wear of rails , equipments and results in saving maintenance cost.  To provide an even and smooth running track to ensure comfortable ride to passengers and safe movement of goods

Equilibrium cant : when the lateral forces and wheel loads are almost equal, the cant is said to be in equilibrium. This equilibrium cant is provided on the basics of average speed of the trains. Cant Deficiency : maximum permissible cant – actual cant Max permissible speed on curve :  Max speed sanctioned on the section  Safe speed over the curve  Speed based on the consideration of super elevation  Speed from length of transition curve.