Note that this is all only in my experience and may vary from what other users experience.
All of my locomotives with traction tires will out pull any without under "normal" conditions. for comparison I'm going to rate things in the toughest to pull cars that I own, K-Like Heavyweight 18" passenger cars. I only have six of these cars, so any number of cars over that is with somewhat easier to pull Williams cars of a similar size. this is on lionel tubular track going around O72 curves, powered by a postwar ZW set with a quality meter to 18VAC. It is worth noting that using these cars is a little unfair, because each one consumes available power with it's two light bulbs. Any mention below of a number of cars is based on these heavy beasts being pulled, but for an approximate number of 'normal' cars, pulling one of these K-line heavyweighs equals something like 4-5 normal cars.
Lionel low end 1970's "sound of steam" 2-4-2 (I think the number on the cab is 8144) (one traction tire): 2 cars, maybe 3 on a good day. An identical locomotive without the traction tire will spin it's wheels trying to pull one of these cars.
Lionel 2055 post war 4-6-4 (Magnatraction): Pulls 4 cars just barely. will pull better if the e-unit is locked, making a big difference on easier to pull cars.
Lionel (I think 8415) early 2000's command ready, upgraded to TMCC, fairly low end hudson 4-6-4 (traction tires on both rear drivers): Pulls 6 cars.
Lionel 2-8-4 lionel lines (737, again if memory serves.)Similar vintage as above TMCC hudson, and pulls about the same. The hudson will run faster, and the Berk has just a little more pulling power. Pulls 6 cars. I'll also add the Lioncheif Polar express 2-8-4 here as well, though I think it should be rated slightly better than the TMCC version as it pulls better at lower speeds.
K-Line 2-6-6-6 Allegheny - TMCC (traction tires on rear drivers on both sets of drivers.): This beast will pull tree stumps for me on straight track. I've pulled all six K-line cars and another 5 williams cars, after which the lights in the cars take their toll on the old ZW. Worth noting that this engine is rated for O31 curves, but it will consistently jump off the track under even a medium load on that tight of a curve.
Lastly, Lionel 'navy' NW2 switcher with the motor in the truck.( traction tires on 2 wheels on same side of powered truck.): This little guy pulls pretty well in a straight line, maybe 3 cars, but will jump the track under any load around a curve. I tried weighing it down and that provided no help. Finally I added a second powered truck and she finally makes it around curves under load, pulling 4 cars.
All that said, the tires are very finicky at times. they will slip, and peal off the drivers if they become oily. In addition, rough track work chews them up quickly. The post war locos don't much care about this, but it is worth noting that some of the track choices out there are not magnetic, so magna-traction becomes useless in these cases
Sorry for the ramble, hope there is something in there you find of use.
JGL
