I'm booking a train for Thanksgiving (which is still cheap for my dates but presumably won't be for long) from Boston-PHL. I don't know whether RTE or BOS/BBY will be more convenient; I expect tickets to be much more expensive by the time I'd know and thus want to change the ticket.

If I book a train from RTE, is boarding at BOS OK? They're the same price, as is usually (always?) the case.

My understanding is that if I book a ticket from BOS and they check all the tickets on the train before getting to RTE, my ticket would be cancelled before I try to board at BOS. Is that correct?

Is the answer any different for BOS/BBY?