This is a great question! I’m not sure what year you are in school yet but in physics we have three really important properties that describe a wave – things like sound waves or radio waves. The waves are like the squiggly lines you might draw for the sea, a bit like an ‘S’ shape on its’ side. These properties are frequency (how many waves per second), amplitude (the height of the wave) and wavelength (the length of one oscillation). Oscillation is a long fancy word which basically means one ‘S’ shape or one up curve and one down curve.
Whilst the vibration of the trains is normally really complex, we make it simpler and use these 3 things to define it. These vibrations are then applied to the soil in my test for a number of minutes (equal to the time taken for a train to pass over the tracks) lots and lots of times to see how the soil behaves under these vibrations.