Track Alignment Design Flashcards
What is standard track gauge?
1435mm
Describe the structure of a rail
- Rail sits on top of a railpad
- Rail held in place on the sleeper by the rail fastener
- On some sleepers a baseplate also exists between the railpad and sleeper
- Baseplates and rail shims* allow the rail height to be corrected; for geometry correction or due to differential heights at transitions
What is the railpad made of, and how stiff is it?
Usually made of a visco-elastic polymer
The pads can exist at different stiffness
What is the primary suspension in trains?
The springs
NB. secondary is the airbag
How is the overall stiffness (ie. the stiffness the train wheel experiences) determined?
It is a combination of the stiffness of the trackbed (sleeper) and the stiffness of the railpad
Define track cant (sometimes called the superelevation)
What is it used to counteract?
The difference between the level of two rails in a curve
“Used to counteract the lateral acceleration (due to centrifugal forces) as a train negotiates a curve at speed”
For standard gauge, what is 2b_o equal to?
1.5m
What does this equation represent
Cant angle
What factors should be considered when determining maximum cant angle?
E.g. if a train stops or runs slowly on a curve
- Passenger comfort
- High lateral loads (and reduced vertical loads) on the outer rail poses derailment risk for freight trains
For a vehicle negotiating a curve (of radius R) at speed v and at cant angle (θ_t), what are the acceleration equations for:
a) centrifugal acceleration
b) gravity acceleration
For a vehicle negotiating a curve (of radius R) at speed v and at cant angle (θ_t), what are the resolved vector acceleration equations for:
a) parallel acceleration
b) perpendicular acceleration
What does a_y and a_z represent?
What cant angle (θ_t) can be assumed as ‘small’ for small angle approximations?
a_y = track plane (ie. lateral) acceleration
a_z = perpendicular (ie. vertical) acceleration
Small angle approximation used if < 0.15 rads
What are the small angle approximations for track plane and perpendicular acceleration?
Assuming θ_t < 0.15
What is the equation for calculating equilibrium cant (h_eq)?
Equation used for a given speed and given radius
For a set cant (h_t) and given radius, what is the equation for calculating the equilibrium (balanced) speed, V_eq
When does cant deficiency exist?
When the cant is less than equilibrium
How can cant deficiency (h_d) be approximated?
Why is cant deficiency applied?
What happens when cant deficiency is applied?
- To ensure a smooth experience for the passenger
- When the cant is lowered (deficiency) it forces the flange of the bogey to run on the outer rail, and is more stable this way
- Bogie will not keep bouncing between one rail and the other, as some lateral force on the outer rail improves vehicle performance
What should be considered when applying cant deficiency?
- The tracks ability to resist lateral forces
- Axle load and unsprung masses; unsprung masses (e.g. axle and wheels) can lead to significant impact forces as they are not damped
- Track alignment (magnitude of track geometrical irregularities)
- Dynamic performance of vehicle
What can occur under high lateral forces (high cant deficiencies)?
What is used to prevent this from happening?
Flange climb* can occur, potentially leading derailment
Especially a risk on tight curves, and lubricants are used to prevent flange climb
What does this equation represent?
It relates the permissible speed (v_lim) to a given permissible lateral acceleration (a_y,lim)
What does this equation represent?
It relates the permissible speed (v_lim) to a given permissible cant deficiency (h_d,lim)
What is the recommended equilibrium cant (h_eq) in Germany?
170mm
How is the curvature of a curve calculated?
By finding inverse of the radius
How does a vehicle running at speed produce lateral forces, and name issues that can occur?
Lateral forces produced by a vehicle running at speed due to the centrifugal forces
Issues include:
- risk of flange climb and derailment
- passenger discomfort
- movement of carriage loads
- risk of vehicle overturning (particularly with strong winds)
For horizontal transition curves, what does a clothoid do?
A clothoid is used to provide a constant rate of turn, to minimise lateral acceleration
If a clothoid isn’t used, there would be a sudden change of direction, causing sudden lateral acceleration
What does this equation represent?
Clothoid equation
As well as using a clothoid, how else does a transition curve provide a transition to canted track?
By using a superelevation ramp
How are rail tracks designed to minimise gradient changes?
- Building bridges and tunnels
- Vertical transition curves
What issues do large gradients have on railway performance?
- Heavy (freight) trains have problems climbing the gradient
- Large gradients need increased power supply
- Traction issues in poor weather conditions
- Higher braking distances when descending
What are the typical vertical acceleration limits for high-speed?
Desirable: 1.5%
Exceptional case: 3.5%
Define lateral track misalignment
What causes it, and give an example of where it can occur
- Caused by lateral loads on a curve that has a cant deficiency applied
- Felt as a large horizontal ‘kick’ by passengers
- If not corrected, can lead to derailment
E.g. on a diamond crossing
Why is extreme heat an issue?
How can the issue be mitigated?
It can cause lateral track buckling
Continuously-welded track can buckle; potential derailment issue
Rail pre-stress is very important in mitigating buckles
What provides lateral resistance in track design?
- Lateral resistance comes from the frictional interface from the sleeper base, crib and shoulder
- Rails act as a mechanism to engage more sleepers in the track lateral resistance due to their bending stiffness (ie. than from just the loaded sleeper alone)
What is the localised track resistance often referred to as?
The ‘track panel’ resistance/stiffness
What makes the most important contribution to total sliding resistance when a track is loaded by a train?
What about for an unloaded sleeper?
The sleeper base (ie. sleeper)
When unloading, between 26-35% for base, 37-50% for crib, 15-37% for shoulder
How can track lateral resistance be improved?
- Increase the interface friction of the base
- Try to improve the shoulder resistance, for example by shoulder end plates (limited success)
- XiTRACK beam devices significantly improve passive resistance
NB. the ‘hump’ on ballast (commonly used in UK) has minimal effect on preventing track panel from moving laterally
(NAQ) difference between carbody and bogie
