Panamint Models on Shapeways has replacement underframes with either stairsteps or side step castings for both the Arnold Rapido Bobber and Bachmann Bobbers. I've not used them but am very satisfied with their CB&Q No.7 and No.2 wood beam waycar and passenger car trucks. They are very accurate models of the prototypes and roll very well. And, to topic, work nicely on the MDC/Athearn Overton/Caboose and MicroTrains Caboose underframes. These style of trucks were widely used (ATSF, MP, UP, SP, NYC, CB&Q and many others) on cabooses and wood passenger cars, many lasting the entire life of the cars. On the CB&Q these lasted almost to the BNSF era with the genealogy of some individual cars stretching over 100 years (although how many pieces of wood were original like George Washington's axe...the handle and the head were replaced several times but it was still his axe!!).
Charlie Vlk