So I would like to make a birthday cake for my twins. I want to base it on this one. Except it will be only one tier, but it will have a roundhouse on it, with James, Thomas and Percy, like shown above. I never really worked with fondant before, how far in advance should i make the trains? Also, if i make the train tracks a few days before, will they dry out? What is best? Thank you!

I've made a similar cake and did all the fondant features the day before and the morning of and just stuck them on with melted candy melts....Unless you were doing a free standing train and not the one pressed up against the side, I'd imagine you can make it whenever you wanted to.  I put the tracks on the morning of, but you could do the day before delivery...

:)

Thanks, the train will be just like in the pic, pressed against the side, just 3 of them. I am just concerned about time, since i have 3 year old twins, that never leave me alone. :) 

AIf you don't plan on eating the trains you can make them as far in advance as you want. I made a 3d Thomas for my son's 3rd and made the fondant face 2weeks in advance it didn't crack at all. If you want the tunnels to fit to the cake surface try laying them to dry on the cake tin you will be using.

You can make the trains as far in advance as you like, just keep them out of direct sunlight so the colors don't fade.  If you want them to stay flexible, keep them in an airtight container. If you want them to dry hard, they will need to be curved around something the same size as the cake (the cake tin perhaps), so that when you go to attach them to the cake, they are already curved (if that make sense) otherwise they might crack if you try to get the flush against the cake..

As for the train tracks, the same applies, if you want them to stay flexible, keep them in an airtight container, or they will definitely go hard and dry out. 

The wooden planks would be fine to dry out though, as long as you keep them nice and flat as they are drying, they will be a lot easier to move and set up on the board if they are dry.  Then you can just add they "steel" lines on top as you put it all together?

I hope this makes sense, I too have 2 young children who don't leave me alone lol