Here is an idea for all to ponder.
Very simple diorama Just a tunnel mouth and bridge sound and smoke.
Now imagine you look at said tunnel mouth ( through a peephole ) It's not a long tunnel and you can see the silhouette of approaching train, sound on of approaching train time about 5 seconds and smoke appears billowing above tunnel mouth, This could be a tube attached to the underneath of the tunnel mouth, for smoke a simple set up using one of those vapour electric fag simulator things just inhale and blow into tube.
Thats it, just sound and smoke, and a single bit of rail, you could add vehicles on bridge or anything you want, and I would make it so the box is diamond shaped as others have done to give more depth...
Feel free to use if you want. or shall I get hat coat