I've seen pigeons getting on District line trains before at Richmond and at Kensington Olympia where they seem to enjoy rummaging through all the detritus - never seen them actually take a ride though, guess they think it's quicker to fly than wait for the underground .
Pigeons regularly used to hop on trains at Edgware Road [Circle] and ride round to Paddington. I once changed ends and fouind one in my cab sitting on the destination plate box. He'd flown in through the open cab door window. Did not seem to be bothered by my presence and sat there quite happily cooing until we got to Paddington then he departed the way he had come.
It seems they had learned that there were rich pickings to be had at Paddington mail line concourse with discarded peanuts and crisps and stuff. Edgware Road was where the roost was.