Heritage railway track and the two second rule?

[_Henry_]

Hi All,

Whilst visiting the heritage railway MHR, I was admiring the gentle(ish) curves of the heritage tracks out of Alton towards Four Marks. I could see the heritage tracks had to do lots of curving and 'weaving' through their points there. Subsequently, when down then line I overhead in the station cafe at Alresford someone talking about the two second rule on railway tracks - saying the train can't go over anything that isn't the same for less than 2 seconds.

Firstly, I thought - why is this? And secondly, surely this '2 second rule' can't be achieved for lots of heritage railways, like the MHR, where you often have lots of old turnouts with small geometry. Surely, these heritage railway would always fails this rule?

I couldn't work this out in my head. Any thoughts would be gratefully received as food for thought.

 

[civ-eng-jim]

Is the two second rule the design standard that says (geometrical) element lengths should be no less than 2 seconds at line speed?

A lot of the national network doesn't achieve that either and over point work it won't apply as you're constrained by S&C geometry.

I've not heard a definitive reason for this rule that I trust! Some colleagues say it's about bogie stabilisation or ride quality but I'm not certain.

 

[zwk500]

I expect its a rule of thumb rather than a strict regulation. It would presumably lead to a smoother ride and lower force transfer which results in less maintenance to track and train. However I don't see how it works with things like transition curves or slip crossovers.

 

[edwin_m]

Certainly in the new high speed line I'm part of the design team for, the layouts specified by the track designers are far more spaced out than is typical on NR.