Can Bachmann's On30 DC/2-6-0 be retrofitted with a DCC decoder? I'd like to equip my layout with DCC control but I need to negotiate 18" curves.
Yes it can.
The only reason a locomotive that can negotiate 18" curves on dc would not negotiate the same curves when fitted with DCC would be if the wiring between tender and locomotive were too stiff or too short.
Note that if you put the decoder in the tender, you end up with 6 wires between the locomotive and tender - 2 for wheel pickups, 2 for the motor and 2 for the headlight. But if you put the decoder in the locomotive, you do not need any wires between the locomotive and tender unless you add pickups to the tender wheels (2 wires) and add a rear light on the tender (1 more wire.) Even then, 3 wires are better than 6. If you are looking at a sound decoder plus speaker, then you are stuck with putting them in the tender.
http://www.pacificcoastairlinerr.com/mogul_dcc/ (http://www.pacificcoastairlinerr.com/mogul_dcc/)