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)

What is it used to counteract?

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?

18
Q

Why is cant deficiency applied?

What happens when cant deficiency is 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)?

What is used to prevent this from happening?

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?

24
Q

How is the curvature of a curve calculated?

A

By finding inverse of the radius

25
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)
26
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
27
What does this equation represent?
Clothoid equation
28
As well as using a clothoid, how else does a transition curve provide a transition to canted track?
By using a **superelevation ramp**
29
How are rail tracks designed to minimise gradient changes?
- Building bridges and tunnels - Vertical transition curves
30
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
31
What are the typical vertical acceleration limits for high-speed?
Desirable: 1.5% Exceptional case: 3.5%
32
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**
33
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
34
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)
35
What is the localised track resistance often referred to as?
The '**track panel**' resistance/stiffness
36
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
37
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
38
(NAQ) difference between carbody and bogie