C - Substructure

Question 1

Explain the concept of track variability

Answer 1

The maintenance requirement for 2 different sections of rail can vary massively, with 2 key factors being the ‘Subgrade’ and ‘Inherent rail shape’ at both locations.

Question 2

What are the properties of a rapidly deteriorating track?

Answer 2

A rapidly deteriorating track will most likely have;
Poor sub-grade
Excessively stressed track - unstresses itself under the action of passing trains.

Question 3

Why is drainage of a rail system important?

Answer 3

Ballast is an unbound material, and no unbound material performs well when it’s saturated (reduced stiffness/strength)

Question 4

Why is non-functioning drainage bad?

Answer 4

Non-functioning drainage can deprive a subgrade of 1/2 its inherent strength, which leads to much more rapid deterioration.

Question 5

What is the definition of Critical velocity?

Answer 5

Critical velocity = Speed of wave propagation

Question 6

Why is Critical velocity an issue in some cases?

Answer 6

In fast trains, train loading (which acts as a pulse load to the ground) becomes foreshortened.

As train speed reaches Critical velocity, nasty vibrations are caused from the foreshortening of the wave pulse (think of resonance).

Question 7

When is Critical velocity an issue that needs to be addressed?

Answer 7

Mostly Critical velocity is not an issue. It is only an issue when;

Track runs over marshy ground - so speed restrictions have to be applied.