2008 mainline layout attempt

[Peter]

Can I ask a naive question. Why do we take de-acceleration of a train to be 0.5m/s/s?

Other than because the examiners tell us to? Do you mean "why do the examiners pick 0.5m/s/s for deceleration?" or "what is the basis of 0.5/m/s/s?"

For the former, I guess it is because it has to be something and, while still requiring you to be able to calculate things, it does not make the sums especially hard.

For the latter, yes, a modern passenger train may achieve 9% or in some cases 12% g braking giving 0.8 - 1.1 m/s/s, but then you'd have to apply a different rate for the freight.

What did you want to use?