[Aloisius]

Orinda => Pittsburg is going the same way and on the same line as Walnut-Creek => Pleasant-Hill, Walnut-Creek => Concord and Pleasant-Hill => Concord (the East-bound Pittsburg Bay Point train). There is no need to get off the train in order to swap tickets.

You are of course right though that you can save far more money if you do get off and swap with someone going the opposite way, but then you have to wait for another train. If you already get off the train in order to transfer (say you commute from Pittsburg to Fremont), then swapping cards with someone going the opposite direction is easy.