Originally Posted by
London_traveller
[...]
I'd second Raffles' point about seat reservations. The other thing you should know is that even if you have reserved seats, you can take other, unreserved seats onboard with no problem whatsoever. This is useful in the exact circumstance Raffles mentions, when the reserved seats are stuffed into one carriage, with the next (unreserved) carriage nearly empty.
Technically, when travelling using an
Advance ticket (i.e. specified train at a specified time), you are supposed to sit in your allocated reserved seat (if there is one) -
however I have never had or seen this being enforced, though I have read of a few occasions where it was (I think on Crosscountry trains).
Ultimately the worst that would happen is that you'd be asked to move to your reserved seat... but in all likelihood that won't ever actually happen!
(Note this never-enforced rule only applies to Advance tickets.)