I've known passing through clapham junction and there were literally too many trains to count. On both sides were trains going towards London alongside our train, and Southern AND SWT units going the other way. There must have been 12-16 movements in the time it took our train to slow, stop and start again. Easily. This was a morning peak at about 8am.
EDIT: in fact, if anyone thinks it's busy out of Euston, Kings Cross or Liverpool st... or anywhere in the UK for that matter - there is nothing like Clapham Junction for a ridiculous number of train movements. However, I may be missing the point, but it's not the most interesting of places... nor the prettiest. Even the trains all pretty much look the same, but if you are of a spotting nature, I imagine you'll need a big pad and a few spare pens.