I made this cake almost identically a few months ago. You can just bake a couple 12" square layers in a firm carve-able cake, and carve your train engine, cart and carriage from these. Stack and fill to create your higher parts.
Use a small flat spatula to apply your ganache.
Cover each part with one piece of fondant, the cut out all your small pieces of fondant to use as the decorations.
It's a bit fiddly, but a fun cake to make!
I have a pic of mine on my FB page.