I've tried this modification to one of my three O2s.
To give you some background my IOW layout features Ventnor and Wroxall stations on two levels. They are connected by an incline - approximately 1 in 40, which snakes around a series of curves (admittedly not ideal, but its worked well until recently). My Bachmann 03 diesel shunter hauls 7 ratio coaches up this gradient no problem. A considerably lighter Hornby 0-6-0 Terrier can manage 4 coaches with ease, 5 with a bit of slipping. In contrast the O2s slow to a crawl as soon as they hit the gradient, and can just about manage 3 coaches at a very slow pace.
Inspired by Chris G's suggestion I've tried adding weight to the smokebox. The body lifted off by removing the two screws in the bunker, then carefully cutting through the pipework on either side of the boiler, and at the bottom of the Westinghouse pump . With the body removed it was very easy to relocate the DCC decoder to the bunker.
I've since experimented by adding strips of lead into the smokebox. Add too much and motor stalls, and the model overbalances, resulting in the pony truck derailing. To be honest I've not noticed any real improvement with the extra weight. Perhaps marginally more grip, but it feels to me as though the motor is just underpowered compared to my other locos. 3 coaches remains the limit on the incline. You can probably achieve the same result without moving the decoder (I've added some lead wrapped in insulating tape into my other two O2s).
The O2s look fantastic - top marks to everyone involved in capturing their look so well. I just wish they had a bit more power - they look a bit out of place with only three coaches in tow.
