Note these motor specs:
24mm square 10-pole rotor motor
3.Motor size: 24*24*30mm
4.Output shaft diameter: 2.5mm
5.Output shaft length: 9mm
7.Voltage: DC 12V-24V
Voltage:12V Current:90mA Speed:4200RPM
Note the ad quotes the 24 volt rpms (8400 rpm) which would be too fast for most larger scale uses..
This is a four magnet motor which is designed to run at about half the RPMs of similar size can motors. Most of these four magnet motors were designed for automotive uses, produced by the millions. When applications
changed, they were dumped on the surplus market by the tens of thousands at perhaps 10% of cost, a real bargain at the moment.
FWIW another Ebay seller was selling this 24mm square motor for 10 cents more, WITHOUT the gear.
Minebea 10-Pole DC 12V-24V Micro 24MM Mini Square Electric Motor Strong Torque
NOTE This Seller Sells them with and without the gear, without the gear is now only Ten Cents Extra!!!-
FWIW there are larger four magnet motors on eBay for use in larger engines, the 24mm square motor is strong enough tor any HO locomotive
HO and Sn42