Hello Bart,

Interesting article and comments. I thought I might offer some observations:

Gearing does not directly affect the maximum load that can be hauled - unless the loco is so highly geared for high speed that the load has to be reduced at low speeds, so as not to overload the traction motors. Electric motors can exert high torque at low revs, it depends on how heavy duty they are as to how long they can do it for. High speed motors are preferably lighter, so are usually lighter duty. Still, modern AC traction motors can have quite a wide speed/traction range.

The number of axles makes no difference at all to adhesion. It is governed purely by the weight on the driving wheels. High speed locos tend to have lighter axle loads to be kinder to the track, while the USA, with heavy track, has high axle loads. High axle loads means less axles for the same weight and tractive effort, which reduces locomotive capital and maintenance costs.

Finally, besides weight, the second major factor affecting tractive effort is the adhesion (wheel slip) control system. Modern freight locos have electronic wheel creep systems, which use radar to compare actual speed with the speed each axle is turning. The control computer then finely manages the power to each axle to limit axle speed to slightly faster than actual speed - this condition is called "wheel creep"). This achieves the highest possible adhesion and so the highest tractive effort. Compared to 1950s and 1960s locomotives, current freight locos, with AC induction motors and advanced wheel creep control, can be scheduled to haul double the loads for the same weight of locomotive!