I prefer the flat green train Danielle. May be you could add a few flat details to break up the bigger shapes. Rivets for example.
The green train has some of the clearest detail to me, the yellow spokes come out nicely compared to the other colour combinations and while that is also the case for the blue train, the green's brighter hue shows the detail more clearly. Maybe you could experiment with different intensities of green to see which sort of level 'pops' the most.
Ah thank you, I've decided to go with the green train, with some yellow designs on the side ^_^