The engine that is less affected by the blowing of the whistle generally has a whistle motor in its tender in a better state of repair, maintenance. & lube. It’s not drawing as much current to operate.
The entire circuit has to be evaluated, though, transformer included.
A 726 with a 2426W tender will draw a lot more current through the rectifier and shunt than a 238 2-4-2 with a 234W tender.