"1934 3rd class open" or "3rd class corridor", but what, please, is a
"corridor brake composite"?
--

Nice and easy - all you do is break down the words in the names of the
coach:

Corridor - It's got a corridor/compartments in it. (as opposed to an 'Open'
carriage - where you've just got seats without any compartments)

Brake - It's got a guards brake compartment (i.e a compartment where the
guard can apply the brake from?)

Composite - The coach is made up of different classes of travel facilities.
(1st class/2nd class in the same carriage).

Hope this helps.