Gear ratios differences have been around for years. The 'sorta' standard EMD gear ratio has been 62:15 - This gave a speed of about 65 mph with 1st generation engines(like the GP9) and 71 mph on most later production. The actual traction motor is the limiter. The maximum rotational speed of the traction motor determines potential top speed. If the motor is spinning too fast at maximum load, the copper winding will 'soften' and fly out - ground short to the motor case. This is why you hear about 'bird nesting' a traction motor. This has been a limitation with DC traction motors. AC traction motors do not have the same limitations.
I remember that BN used 60:17 for most of their merchandise SD40-2 fleet at one time, but had 61:16 for dual service merchandise/coal SD40-2's The faster the top end is, the high the minimum continuous speed is(and this affects the 'short time' rating). I am not sure what the current gear ratio information is for the BNSF flee
Ok here are the EMD gear ratios and top speed for each based on pre D77 traction motors and 40" wheels.
65:12 gearing 55 mph. used on a lot of early switch engines and some early road engines.
62:15 gearing 65 mph by far the most common EMD gear ratio for road locomotives
61:16 gearing 71 mph used by roads wanting a little faster speed and on some dual service engines
60:17 gearing 77 mph. used by BN and others for fast freight
59:18 gearing 85 mph used by UP on the DDA40X and "Fast Forty" SD40-2s used by newer pass.
58:19 gearing 92 mph fairly rare but some use made of this ratio
57:20 gearing 98 mph. fairly common passenger gearing for E-units with 36" wheels giving 91 mph
56:21 gearing 105 mph but most common on E-units giving 100 mph top speed
55:22 gearing 115 mph common E-unit gearing giving 110 mph speed with 36" wheels
When the physically stronger D77 and D87 motors appeared top speeds were bumped up about 5 or 6 mph.
Most Amtrak EMD locomotives (SDP40F and F40PH) had 57:20 gearing and were set up for 100 mph maximum. A small pool of F40PH locomotives received 56:21 gearing and a 110 mph max. speed for eastern service.
Common GE gear ratios were 74:18 for freight service, and its "Fine Mesh" replacement 83:20. IIRC a common dual service GE ratio was 65:24 giving an 85 mph top speed. This would have been found on RS3 and RS11 locomotives set up for secondary passenger service.
Gear ratio does affect tractive effort so higher speed ratios lower the T.E. You can mix and match ratios as long as you understand that those locomotives with higher ratios also have higher minimum continuous rated speeds, and those with lower top speeds must not be operated faster than their top speed. In practice a little difference is not a big deal, but using locomotives with a big difference is likely to cause trouble.
Just to clarify-- when you compare two engines, identical except one has 56:21 and the other has 65:12, the latter engine will have a higher maximum TE. But at a given speed (above, say, 20 mph) the two engines will pull about the same.
Probably more correct to say that the latter (65:12) engine will have a higher continuous tractive effort. I would be surprised of most locomotives with 56:21 gear rations have the maximum short term tractive effort limited by adhesion.
A EMD D77B traction motor produces 4850 lbs of torque as it starts to rotate. Assuming 40" wheels the T.E. at the rail would be the following
62:15 20,030 lbs.
61:16 18,478 lbs
60:17 17,121 lbs
59:18 15,908 lbs
58:19 14,792 lbs
57:20 13,822 lbs
56:21 12,949 lbs
Chances are that a higher-geared engine will have the same starting tractive effort as a lower geared engine - which would typically be limited by adhesion. You just have to feed more current into the traction motor, but that means that the time that a higher-geared engine can produce adhesion limited tractive effort is substantially less than what a lower geared engine is capable of.
