Larry says:

A bobber is the generic name for a short 4 wheel caboose that did not have trucks, but rather a wheelset on each end. The various forms of bobbers may or may not have had cupolas, if it did have one it might have been in the middle or at one end. They were used all over, but were especially popular when the trackwork was rough, usually on logging roads, or shortlines.
I don’t know how you would find out which roads used them, it seems like all of them had one at one time or another.