Two comments.
Tap in tap out on trains would destroy dwell times. The authenticator would have to be right by the door so anyone with luggage, bike, pushchair would have to either load everything into the vestibule, then faff about getting card out and return it (have you seen how long people at checkouts can take finding cards from wallets/handbags?!?) then gather up all their stuff before the next person can board. And the safety risk for children would go through the roof as parents have to let go of them at the most dangerous point of a train journey. And how many cards are going to end up going down the platform/train gap?!!
It’s common practice to have different prices for different payment types (DD discounts etc).
Wouldn’t the quickest pay train method to be that the guard only sells singles to the end of the line or the biggest interchange place, and those singles cost a pound or two more than e-tickets or TVM?
Less faffing for guard and incentives for people to make the effort to buy before boarding.