Originally Posted by serpens

That is my understanding and what people on another board have mentioned. To expound, a person was in that situation and booked a room from A to B and a different room from B to C. This was accomplished using the multi-city option at amtrak.com, and there was no discount for traveling from A to C; it was the offered price from A to B and the offered price from B to C, which made it an expensive trip.