You can no doubt do it with other folding bikes, but we have Bromptons so thats what we used. With a grumpy conductor/driver the smaller your bike looks the better luck you'll have. Laura had problems with the station manager at the Albany station a few months ago, but managed to get on.
It was a short trip so we took the bus from Portland to Albany about 2 hours and rode for about 2 hours. All in all, a good trip that was made considerably easier by the fact no boxing of bikes was necessary.