The material of the of the train wouldn't really matter, William can create quicksand from Anything as long as he can determine the chemical compound. 

It makes sense that it would create a hole in the train . Steam powered trains have quite a bit of bulk underneath the floor though. The quicksand would lead to a bunch of pipes and bars, but you could probably see the tracks through those.

If you were wondering whats holding the quicksand up, well, I have no explanation. You could say its part of my alchemy to slightly manipulate the quicksand.

So basically you'd sink until your feet touch one of the bars or pipes that connect the wheels and stuff.