What motors and gearboxes can be used?
(updated 02-06-16)
Mashima open frame motors (12/16 or 12/20) have long been used. 14.2mm users can often, with advantage, use the more powerful, slower and smoother Mashima 1220 flat can motor (or sometimes even the 1224); the 1220 can be squeezed into some of the larger 12mm locomotives. The smaller Mashima 1015 and 1020 flat cans can also be used; the 1015 has a mixed reputation but the 1020 seems a good motor.
The Branchlines RSL gearbox is an old favourite, and has the advantage of a grub screw, useful if you think you may want to pull things apart. Branchlines 2-stage gearboxes have also been used. However, the Branchlines gearboxes require careful checking, assembly and running in to work well. High Level multi-stage gearboxes offer a quality alternative which is easy to assemble. The range includes the Slimliner+ and Slimliner Compact+ specifically designed for 3mm. Also there is a Drivestretcher available for us with each of these (D3 and D4 respectively).
For diesels, a variety of approaches are possible. 3SMR stocks Bullant motorboxes, amongst others, or will obtain them to special order. The Society produces some motorising kits which may be suitable.