In article ,
Roland Perry wrote:

The ticket will always be valid on a direct train (one which calls at
both the start and finishing stations)

....unless there's a negative easement that says it's not.

I didn't realise that exceptions to the through train rule were
possible, but there is apparently a (fairly new) such easement for
some loop line (I'm sure others can fill in the details) to prevent
you from going the long way round the loop even though it's a direct
train.

-roy