Everyone raves about the 8-wheel worm drive of the 736. But my own experience with them isn't that great... The motor is geared only to the rear-most axle, and power is transmitted to the other wheels through the drive rods. If the rods aren't lubricated, the holes wear wide, causing erratic operation. The bushings of the rear axle also tend to wear in an oval shape, which causes the rear wheels to rub against the frame, and binding. Even after substituting a larger 25-tooth worm wheel in 1948, the gear ratio is only 8.33 to 1. This is pretty tall, which gives a high top speed, but not very much low speed torque.
Meanwhile the 2046 and 2035 have a spur-gear mechanism similar to the prewar 226, albeit with lighter-gauge aluminum sides and magne-traction. The gear ratio is about 11-to-1. That means more RPM and more torque. The spur gears do a better job of transferring power equally to all six wheels. Pulling power is mostly about the weight (and on steel rails, magnetraction) but unless the 736 is much heavier, I would put my money on a 2046 or 2035, especially on tight O31 curves.
As Lionel moved away from 1950, I do believe that tooling got worn, and manufacturing tolerances declined. So a late-1950s 646 might not run as quietly as the original 2046. My $.02
