It should be noted that some early electric and diesel electric locomotives (especially the early AC stuff on Southern) used repulsion motors instead of brushed DC motors.
That would mean you would have an alternator coupled to the motor field windings and the brushes would only carry low voltage armature current - with the attendant reduction in sparking and brush attrition.
Repulsion Motors are also rather easy to control, and you can do the drivetrain without rectifiers, which was important before they existed and your only real alternative was the universal motor - however they have serious problems attempting to do dynamic braking, it can get a bit hairy as the voltage across the alternator climbs rather alarmingly.
