Most buses have CCTV. All TFL buses do, and also lots of routes operating out of London. I know. Spent few years as a London Bus driver. Retired July. Every bus has 14 cameras recording continuously. Four outside. Front, back, nearside, offside. All the others various places in the bus. While driving, the driver cannot monitor all at the same time. The picture on the CCTV screen in the cab shows the upstairs view all the time until the driver opens the exit doors at a bus stop. Then the picture changes to the exit doors. Once they are closed, reverts to up stairs view. Should the need arise to view the other cameras, the driver has to stop the bus and push a button on the CCTV control panel. Then it scrolls through every camera on the bus. But of course, shows real time, not what was happening earlier.
If someone was was doing something like indecent exposure, or the like, and a complaint was made, the CCTV can be downloaded onto a disc once the bus returns to the garage. Anyone identified can be charged with "an act of indecency in a public place". So if you were shagging up stairs and a complaint was made, the police would have to identify you from the CCTV. The chances of getting caught are slim. Too many one-off crimes happen on buses where the suspect is not know to police and can't be traced. As for trains, all LUL trains now have a CCTV camera each end of the carriage. The driver doesn't have access to view any. So if you were shagging in front of other passengers, and a complaint was made, same as the bus. CCTV downloaded when the train returns to depot. All in all, you can shag as much as you like on public transport. But be warned. All police forces in the UK have free transport on buses and trains. So while your happily shagging away, a copper be it on or off duty could be watching. Then you get your collar felt