I got a full refund for an Acela ticket the week after I travelled. My wife, 3 month old son, and I were taking a weekend trip from BOS to NYP. Since Acelas frequently sell out, we wanted to make sure that we had an available seat to put his car carrier on. The ticket agent let me know that the conductor would (99%) not ask for a ticket for my son, even if he was occupying a seat. I ended up buying him tickets just so a) no one would be denied a seat and b) so I had something to show anyone that complained. The agent said that there should be no problem getting a refund for any unused tickets.

As it turned out, the conductor didn't want the tickets (and looked at me like I had 4 head when I offered) and I went back to the South Station ticket counter the following week and got a full refund.