Brian, the "flat car" behind #11 is actually the frame and running gear of it's tender. No. 11 was used to power a steam pumping engine for the water tower at Whitney, but that use was discontinued when the tank was moved to the Camp Creek crossing. When the locomotive was donated to the university at Pocatello it had a complete tender, but I don't know if it was the original one.
The idler car has been identified as the frame of a Baldwin tender but we don't know which one.
We have the remains of the cistern from an early tender at McEwen, but again we don't know from what locomotive. It was recovered from along the Middle Fork of the John Day River, though not in the river, but is in very bad condition.