Track moving up and down as train goes over.. - Trains Magazine

As an aside - ever sat on a bridge, with traffic moving on the other land(s) - a whole lot of shaking going on.  When you are driving over the bridge - it still has all the vibrations & shaking, you don't feel it.

Transient loads have to be handled, no matter the structure, when the loads are great enough the can be seen and/or felt.

It's the same in concrete parking garages with multiple stories, especially the top outdoors floor, say at a shopping mall.  Felt like an earthquake if a 5 ton delivery truck happened to move two or three rows down.