The roaring of the tube trains over certain (ancient) sections of the track, (Todays news flash - a rail broke in the Tube today, and they've chained it together (!) and made the trains run at 5 mph. You couldn't make this stuff up.)
The distinctive smell in the deep tubes: iron (from the steel on steel) and low oxygen content (pre-breathed).
Everyone in the car, swaying in exact unison in the tube trains as they go over that old track (alright, that's a sight).
The musical 'ching-chong' we hear with every train that leaves Victoria, as they negotiate a certain stretch (probably points) - we're right by the Elizabeth Bridge, over the tracks out.
The brilliant blue flashes from those trains as the pick-ups arc over joints. Like lightning, and it probably plays hell with electrical devices close by, too.
Buskers in the tube tunnels - snatches of music that's generally very high quality - blues, sax, a capella are the favourites. The sounds carry for ages and can be very eerie.