Multiple trips on the same cross London transfer? I cant see anyone being bothered with needing or wanting to do this in any case, and if taking other buses then the driver or conductor would pick it up on boarding
It probably wouldn't be a problem on day returns or any other rail ticket valid for only one day, because the maximum Oyster fare for unlimited travel on TfL buses is less than £4. I doubt there are many (any?) cross-London rail tickets that can be bought for that price, so there would be very little reason for passengers to cheat with those tickets.
However imagine the cheating potential if you have a return valid for a month. Unless bus drivers were issued with some ability to stamp the date on a rail ticket, you could theoretically have passengers using the same rail ticket to get a full month's free travel on central London buses, with no means for anyone to detect it.
Having said that, if you could provide bus drivers with some means to mark tickets to prevent fraud, I too think that allowing cross-London transfers on buses would be a great idea.
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Train operators may be worried that if someone gets the wrong bus or the bus is snarled up in traffic they'll end up claiming under Delay Repay. It might be difficult to refute such a claim as the passenger might be uncertain which bus they were on etc.
I'd imagine that could easily be solved by stipulating that delay repay does not apply to delays on cross-London buses, when passengers chose to take a bus knowing that a train was available.