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RTID > Track Alignment Design > Flashcards

Track Alignment Design Flashcards

1
Q

What is standard track gauge?

A

1435mm

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2
Q

Describe the structure of a rail

A
  • 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
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3
Q

What is the railpad made of, and how stiff is it?

A

Usually made of a visco-elastic polymer

The pads can exist at different stiffness

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4
Q

What is the primary suspension in trains?

A

The springs

NB. secondary is the airbag

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5
Q

How is the overall stiffness (ie. the stiffness the train wheel experiences) determined?

A

It is a combination of the stiffness of the trackbed (sleeper) and the stiffness of the railpad

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6
Q

Define track cant (sometimes called the superelevation)

A

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”

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7
Q

For standard gauge, what is 2b_o equal to?

A

1.5m

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8
Q

What does this equation represent

A

Cant angle

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9
Q

What factors should be considered when determining maximum cant angle?

E.g. if a train stops or runs slowly on a curve

A
  • Passenger comfort
  • High lateral loads (and reduced vertical loads) on the outer rail poses derailment risk for freight trains
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10
Q

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

A
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11
Q

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

A
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12
Q

What does a_y and a_z represent?

What cant angle (θ_t) can be assumed as ‘small’ for small angle approximations?

A

a_y = track plane (ie. lateral) acceleration

a_z = perpendicular (ie. vertical) acceleration

Small angle approximation used if < 0.15 rads

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13
Q

What are the small angle approximations for track plane and perpendicular acceleration?

A

Assuming θ_t < 0.15

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14
Q

What is the equation for calculating equilibrium cant (h_eq)?

A

Equation used for a given speed and given radius

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15
Q

For a set cant (h_t) and given radius, what is the equation for calculating the equilibrium (balanced) speed, V_eq

A
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16
Q

When does cant deficiency exist?

A

When the cant is less than equilibrium

17
Q

How can cant deficiency (h_d) be approximated?

A
18
Q

Why is cant deficiency applied?

A
  • 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
19
Q

What should be considered when applying cant deficiency?

A
  • 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
20
Q

What can occur under high lateral forces (high cant deficiencies)?

A

Flange climb* can occur, potentially leading derailment

Especially a risk on tight curves, and lubricants are used to prevent flange climb

21
Q

What does this equation represent?

A

It relates the permissible speed (v_lim) to a given permissible lateral acceleration (a_y,lim)

22
Q

What does this equation represent?

A

It relates the permissible speed (v_lim) to a given permissible cant deficiency (h_d,lim)

23
Q

What is the recommended equilibrium cant (h_eq) in Germany?

A

170mm

24
Q

How is the curvature of a curve calculated?

A

By finding inverse of the radius

25
Q

How does a vehicle running at speed produce lateral forces, and name issues that can occur?

A

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)

26
Q

For horizontal transition curves, what does a clothoid do?

A

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

27
Q

What does this equation represent?

A

Clothoid equation

28
Q

As well as using a clothoid, how else does a transition curve provide a transition to canted track?

A

By using a superelevation ramp

29
Q

How are rail tracks designed to minimise gradient changes?

A
  • Building bridges and tunnels
  • Vertical transition curves
30
Q

What issues do large gradients have on railway performance?

A
  • 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
31
Q

What is the typical vertical acceleration limits for high-speed?

A

Desirable: 1.5%
Exceptional case: 3.5%

32
Q

Define lateral track misalignment

A
  • 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

33
Q

Why is extreme heat an issue?

A

It can cause lateral track buckling

Continuously-welded track can buckle; potential derailment issue

Rail pre-stress is very important in mitigating buckles

34
Q

What provides lateral resistance in track design?

A
  • 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)
35
Q

What is the localised track resistance often referred to as?

A

The ‘track panel’ resistance/stiffness

36
Q

What makes the most important contribution to total sliding resistance when a track is loaded by a train?

What about for an unloaded sleeper?

A

The sleeper base (ie. sleeper)

When unloading, between 26/35% for base, 37-50% for crib, 15-37% for shoulder

37
Q

How can track lateral resistance be improved?

A
  • 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 improves passive resistance

NB. the ‘hump’ on ballast (commonly used in UK) has minimal effect on preventing track panel from moving laterally

RTID (7 decks)

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