The SunXCD rear hub allows you to run derailleur gears on a frame with a 120 mm dropout width. It features a short freehub that takes Shimano cassette cogs - either 6 cogs from a 7 speed cassette or 7 cogs from a 10 speed cassette. Depending on the cassette a spacer is normally needed between the hub and largest cog so the lockring can be correctly tightened. You'll most likely run downtube friction shifters but the right selection of cogs and spacers would mean index shifting is possible.

So why do Hubjub stock a hub for derailleur gears? Well here's the thing. Apart from the fact that I think they are beautifully made (I've got one for my old steel road bike) all you need is a SunXCD Chain Tug with Deralleur Hanger you can run multiple gears on your 120 mm OLD track frame.