Yay for train games !
The sound of yours is well polished ! especially when the wood logs hit the metal in the train, it's nice that you though about these details.
leveraging the physics engine is quite a good idea as well, I could definitely see a full game based on this.
The one detail I noticed that could even more improve the immersion is to make the sound of the train on the tracks mono instead of stereo, the one that we ear when the train goes fast enough. with a headset we can hear the boggies go from right to left which is a bit wierd as the train stay in the center of the screen. If that can be of any help, to make the same effect in our game, we put the track in mono and used granular synthesis to have it randomized a bit.
Anyways, this is a very solid entry, great job !