Many older locomotives with mechanically-commutated DC main generators use the generator itself as a starter motor. I'm still learning my way around the ALCOs, but the EMDs have 6 separate field windings in the main generator - one of which is a low voltage start winding. In that case, when a nominal 64 volts DC is applied across the start winding the generator behaves just like a DC motor and develops enough torque to crank the engine. I believe I have a video of the main generator cranking over an EMD SW-1 somewhere on my channel.

As I understand, the only reason modern locomotives don't still do this is because they use AC main alternators with built-in rectifier bridges instead of true DC machines. While improving power density, energy efficiency and reducing maintenance, this also makes a "generator-start" arrangement impractical.

@@heavy-iron thanks for your elaborate answers and the videos. Living in Europe I'll never see a 539T live but these videos and the excellent sound surely brings a very close experience to the real thing.

Instead of using the traction part of the same alternator, modern locomotives will crank with the three-phase companion alternator (and whatever GE calls it) to get the engine to rotate. It uses a crank inverter to crank the engine. The companion alternator field is a separate group of windings in the main alternator, similar to generator start windings.

Ge 45 tonners also be like that​@@heavy-iron