DT's. Has to be.
Or perhaps The Boy dropped something in his tea... before locking him in like?
You could be right. I think with the DT's it is massive fcuk off insects rather than trains full of cakes type visions.
Case Solved!
1. Boy drops unknown chemical substance into Nick’s tea and runs outside.
2. Nick enters bizarre hallucinogenic state and can’t find the back door.
3. Nick falls out of window and bangs head
4. Muttering incoherently, he imagines being in a bun shop with a train set (or train shop that sells buns).