To make the tunnel, cut the 2x10 piece in half, put one half on top of the other then carve the top into an arch.
You can make grass by pushing green fondant through a sieve if you want to decorate the green bit some more, or pricking it all over with a fork also gives a good grass effect.
OR, even simpler, if you look on my profile there is a tent cake.
You could do one similar to that. The tent can be the tunnel and you can do a Thomas model or toy instead of the people.
It's an 8 inch square cake (shout if you want a good, really easy recipe that stands up to being cut without crumbling) which you can cut in half, stack one on top of the other, sticking them together with butter cream, carve the top into an arch, cover with grey icing, make bricks out of darker grey and put all over the top/sides of the cake. Cut an arch out of black icing, stick to the front and put the Thomas model in front of it.
If you want to ask any thing, just ask. I'm going to bed in a minute but will be on and off here most of tomorrow.