Double-deck trains are widespread in continental Europe. The best of them are in Finland, where the railways are to the Russian gauge and vehicles much larger than everywhere else. The upper deck lacks the usual uncomfortable curved windows found on double-deck trains and there is a proper luggage shelf above. (photograph)
The only real problem is that there is a shortage of space for large cases, which have to be dragged up the stairs. There are lockable luggage lockers but these will not hold a large case.
The benefits of double-deck trains are marginal, due to the space occupied by the staircases. Much time is taken loading and unloading at stations. These are also quite heavy vehicles with an axle-load of 12.5 tons, which is a lot for a trailer, compared with the 9 ton axle-load of a mark 3.
If we are expecting to have double-deck trains in the future, this must be planned for at the outset. The railway must be constructed to a larger loading gauge than the present continental standard, as we should not be constrained to having vehicles with low curved ceilings.