How can I bump in to 10 trains in one run?

Get close to the train but don't hit it then change lanes and you should run into the corner of the train. You will know if you have because the police will catch up to you but not catch you. Wait until the police slows down and is off your screen to do it again.

Actually, bumping into trains is much easier than bumping into barriers. All you have to do is, while running past a stationary train car (or while a moving train car is passing you, but this is more dangerous), while it's side is right next to you, swipe in it's direction. No real timing is involved. You will bump into the side of the car and the cop will catch up to you for a while. When he falls behind again, you can do it again without being caught. Just make sure you don't have to jump, roll, or (especially) change tracks immediately after you bump a train, as the game will freeze your controls for an instant.