Why does a large train cause the ground to shake?

There's a nice article on exactly this subject in this PDF file.

Summarising from the article: the vibration arises because the track is not completely smooth and the train wheels are not perfectly circular. As the train moves along thetrack, the result is an oscillating force at each wheel/track contact, and this is transmitted to the ground at each sleeper/ground contact. It's this force that shakes the ground.

With the sleeper question confirmed, I'm pretty sure I know what happens. friend of mine wrote his thesis on rail vibrations.

Basically, the rail sags between sleepers. Not a lot, normally, but the sagging is quite periodic. John Rennie got the basic assumption right about rails not being straight, but the wheel issue probably doesn't cause earth vibrations - the wheels are out of phase with respect to each other. (They're noisy, though. Modern trains have ABS for this reason). But that periodic sagging of the rails over the sleeper, excited by the train passing over will add up. Those vibrations will cause that earth shaking.

Modern high-speed tracks are moving to ballastless track beds, without those sleepers. Trains bounce a lot more at 250 km/h than at 25 km/h. Even so, the Dutch can't run their high speed train at 300 km/h; the vibrations would cause the track bed to sink in the muddy soil.