This is yet another ZephyrCab physics post. It's worth noting that this post's math is almost entirely theory, basically cobbled together by me for the sake of this software. So take it with a grain of salt.
If you don't know, diesel locomotives have an ammeter in the cab to indicate load on the traction motors. This is simply a meter that measures the electrical current (in Amps) going to the traction motors.
But how do you simulate one of these ammeters?
If you're a bit of an electronics hobbyist like me (a rather unskilled one, I'll admit) your first thought might be to take voltage and some other data and try to algebra your way to current. The problem is this gets very complicated, and in my experience it's essentially not possible to obtain the data necessary to calculate it this way.
The answer? It appears amperage is directly proportional to tractive effort.
Tractive effort is the output force (in pounds or Newtons) at the wheels, and I've written ZephyrCab to calculate it using a shockingly accurate estimation equation from a Virginia Tech engineering paper.
Amperage follows the same curve that tractive effort does (or close enough for these purposes). We know the beginning point for amperage by looking at the locomotive's data sheet and finding "Starting Tractive Effort." For the EMD F7A it's 56,500lbs. I also know the maximum amperage for the F7 is about 900 amps. So the math looked like this for me:
(Current Tractive Effort / Maximum Tractive Effort) * Maximum Amperage = Current Amperage
This seems to produce reasonably realistic numbers, and I'm planning on integrating it into ZephyrCab here shortly. Hope this helps!