I appreciate your faith in Science, but I don't think I have anything here, I'd need a seismometer or something, and chemists don't have those.

I think it is the trains, though. I counted this morning and there's six bridges carrying the lines across Seven Sisters Road there, with some single width and some double, suggesting about nine lines - the trains could easily be every couple of minutes, when you count the services going in and the ones going out and the ones only going to Cambridge or Kings Lynn and the ones going all the way to Scotland and everything inbetween. Sometime when it's warmer I'll have the (double-glazed, fairly sound-proof) window open when it's quiet at night and be able to double check by noises.