By "AC", I assume you are referring to a loco with a traction alternator (EMD AR or GE GTA/GMG) instead of a main generator.

EMD went to a pair of big Delco starter motors, that depending on the loco are in parallel or series. I have heard that some Alcos used air starts, which I believe is also common in marine use. EMD may now be using a combination of air and electrical starting motors, as the AESS systems (Automatic Engine Stop-Start) are hard on starters. GE initially used a pair of starters geared to the traction alternator, one of them doubling as the auxiliary generator. I believe GE now uses a type of capacitor discharge system to motorize the traction alternator for starting. It is kind of wierd, you push a button and wait a few seconds until the engine starts. I have heard the gen-sets used a stanard starting arrangement (24v?), but the constant cycling has caused most customers to request a capacitor discharge system.

Just remember - to every rule there is an exception and I am not familiar with the real modern stuff, which probably requires an I-pad to start them.