If you have an oddball project in need of a super-slow, low-level drive, give consideration to this MicroMiser.
Hang it off the rear axle of a Colonel Stephens style railbus, or use it to power a 7mm Jeep Train, with the motor sitting low and out of sight in the bonnet space - you can even use the remote attachments to replicate the driveshaft/differential arrangement of the real thing.
For these types of applications, you won't need a high top speed, and space for a motor is likely to be limited. Couple the MicroMiser to a small 1015 motor and you'll still get excellent low-speed performance using the high ratios on offer.
Output is via a 2mm axle but you can can easily bush this to 1/8in.