I work in a 4 story building that is approx. 150 feet away from a set of train tracks. When a large (40+ car) freight train goes by, the shaking in the building is perceptible. As I've watched the train go by, there does not appear to be any side to side movement and the speed is constant. What exactly is causing the vibrations in the ground? Is it simply the train's traversal over each segment of track? Surely the train itself is not "bouncing" along the track with enough energy transfer to shake the ground, right?
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.
According to my research into the vibrations of passing trains, flat spots on train wheels can be caused by skidding train stops. This flat spot causes vibrations. Lack of rail inspection and maintenance can cause vibrations.
We have trains with no vibrations go by so I tend to believe our vibration problems are caused by bad wheels. Due to the fact that some of the heaviest trains going by cause no vibration, again, I believe the vibrations are because of bad wheels. Trains pick up freight from many companies and so of those companies to little to no maintenance. It seems to me we need to for some kind of alliance that makes wheel inspection mandatory.
Since "enough" information is not available, one can only guess. My guess is that "most likely" resonance is what's causing the perception. The length, height, width, and composition of the building and its distance from the tracks, determines its "natural" oscillating frequency and the train's length and speed must create an oscillation that closely matches the building's frequency (or its harmonics), so that the small train vibrations get amplified to the point of being felt in the building. In order to find out if this is the case, there should be a train length that causes the "most noticeable" shaking. Longer or shorter than this, should produce less noticeable shaking.