This article is from the Model Trains FAQ, by Christopher D Coleman with numerous contributions by others.
This depends greatly on what type of trucks your stock has. Newer
(1971 and up) cars usually have needle point bearings in low friction
plastic which allows them to roll very easily. Older cars have no
bearings at all and take 2-5 times more force to roll and are heavier.
These are estimates of pulling capacity based on drive train:
* Dual DC can motors, spur gear: 8 old, or 20 new
* Single universal motor, spur gears: 15 old, or 35 new
* Single universal motor, worm gears: 22 old, or 45 new
* Dual universal motor, worm gears: 35 old, 60 new
Magnetraction and rubber traction tires can, of course, increase the
pulling capability of an engine. Magnetraction is superior in gripping
and also grips with all powered wheels without insulating them from
the track as tires do. Magnetraction is, however, far more difficult
to replace if it fails.