I don't know the answer to your torque question, but I do know that O-scale is split into two distinct varieties.
The most common, at least on this forum, is three-rail. This type of O-gauge train is the classic Lionel type with a third rail down the center of the track. These trains run on AC power from a "transformer." The transformer simply steps down 120 volt AC house current to about 18 volts AC to operate the trains.
The center rail is the incoming power supply to the locomotive, and the two outer rails are power return from the loco back to the transformer.
O-scale two-rail uses the same DC power system as HO-scale & N-scale. These trains are controlled by a DC "power pack" which contains both a transformer, and rectifier, & possibly other electronics. The transformer again steps down 120 Volts to a safer level, and the rectifier converts AC to DC power for the DC motors in these trains. One of the two rails supplies power to the locomotive, and the other rail acts as the return path to the power pack.
A new control system called "DCC" (Digital Command Control) has entered the hobby, and is very popular. It can be used with HO-scale, N-scale, O-scale two rail, Z-scale, & any other scale using DC motors. Lionel makes a similar system for O-gauge AC three-rail too. These control systems let you run two or more, trains on the same track easily. They require far less wiring and also have optional sound from the locomotive.
I was under the impression that "can" motors are DC or can run on either DC or AC, whereas the Lionel "Pullmor" open frame motors are AC only. When MPC/LTI used can motors in their engines they were often listed in their catalogs as able to run on either AC or DC. After a while they stopped listing that, not sure why.
Just the opposite. Can motors with permanent magnets are DC only. Pullmor/Universal motors will run on DC or AC. They have a commutator and wound field. The field changes polarity at the same time the armature winding does so they always repel. Something the permanent magnet types can't.
A DC can motor is a natural transmission. As load increases a DC can motor will trade reduced speed for increased torque. Not withstanding a control system, that's a prime reason for good low speed operation.
The classic Lionel AC motor is a universal motor. Universal motors also exhibit the same speed-torque trade off. But not as well as a DC can motor.
Perm-mag Motors run only on DC, however the on board electronics, either a command control board or reverse unit convert AC to DC. Perm-mag motors are more efficient and less expensive to manufacture than (Pull-More) series wound motors.