Graeme likes the cupcake train idea. Two very easy alternatives for creating a train cake your kids will love. One is a Thomas the Train cake in the shape of their birthday number, and the other is a cupcake train.
The Thomas the Train snack train made for third birthday party. It's just two packs of aluminum loaf pans from Walmart (mine came with plastic lids-great for leftovers), a roll of black electrical tape and of course, Thomas.