>>1105250a bit like people watching but with trains, go to a location of your choice, such as a station, bridge or level crossing and just watch trains go by. Many often take a camera with them to create a photographic record of all the trains they see.
if you wanna go hard core start collecting numbers with the aim of seeing every running example of a specific type.
personally I've only done trainspotting once and that was when a nearby station was about to bin their semaphore signals dating back to the 40s with un-charismatic electric light signals. I also managed to bag some rather spectacular shots as the sun went down and backlit the signals and eastbound trains (see picture provided)