In terms of factory made diesels, I believe Super Scale made a NW2, there are currently NW2 and GP38 shells available, Ken Kidder (might be Kemtron actually) made a GP7/9, and Cussawago Valley/M. P. Davis made a set of cast bronze F units. Hallmark made a set of F3A units, unpowered, as 60% sized Lionel units, but without motors. AHM made a diesel switcher, 25 ton Plymouth I believe, that was HO gauge, but O scale. Older Atlas models are close to OO scale, but Ho gauge. John Ericsson nows more than I, he runs American OO Today.
http://www.americanoo.blogspot.com/