You can get a couple of days out of them as long as they are in an airtight container and stored somewhere quite cool. I know people have frozen chocolate crackles, but can't do with honey joys.

The cake bits of the train cake can be made ahead. You can freeze the cake, just make sure it is wrapped in cloth (muslins are perfect for this) when it goes in, and that it stays in the cloth until it is fully thawed. I think you can make buttercream icing in advance, but it only takes 5 minutes so I wouldn't bother.

Good luck! Sounds like fun!