Proof of payment fare collection has been in place, officially, since 1990 on the Queen route. All riders are expected to have a valid pass or transfer and in return they can board at all doors. Riders who pay a fare when they board are given a transfer in case a POP inspector comes by.
This setup allows all-door loading at major stops, shortens the time lost to boarding and gives a better distribution of passengers through the cars. This is important on the ALRVs which ply Queen Street.
In practice, this does not happen very often. Moreover, many operators don’t open all doors at most stops and the benefit of POP is lost.