The farts you encounter before an imminent turd, not unlike the warm breeze felt when waiting on the platform of a tube station as a train approaches. The "train" in this case is your stool, & the tube is your anus.
Therefore, I can only assume that the correct phrase is in fact 'Changing At Baker Street". As at Baker Street station it IS possible to change between the Hammersmith And City (pink) and Bakerloo (brown) lines.
Last night I went over to Matt's and he told me to shove the end of a toilet roll tube into my arse. Once I'd done this he paced his anus on the other end and we proceeded to poo into each other. I asked him what we'd just done and he told me we'd just been commuting the Bakerloo line. We'd not even left the house.