I tried Sarah:
You asked: can I take my dog on the train?
Sarah says: Dogs and small animals may be taken on board our trains, however all animals, except dogs, must be in a fully enclosed basket or pet carrier designed for this purpose with dimensions not exceeding 85 x 60 x 60 cm. Baskets and pet carriers must be large enough to allow the animal to stand and lie down in comfort. If animals are too large for a basket or pet carrier with dimensions 85 x 60 x 60 cm they may not be taken on the train.
You asked: dog on sleeper?
Sarah says: On sleeper trains, only assistance dogs are allowed in the sleeper cabins. Other pets must travel in the guard's van in a suitable pet carrier.
Sarah also told me that I can give my dog water and feed it on the train, but that must not be done in such a way that other travellers will find it a nuisance, and that I would have to provide the water and dogfood myself.
Hmmm ...
I'll be interested in the answer you get back - Sarah is woolly - I
think you'll be fine, but that's an educated guess.