>The series-parallel to parallel transition was still used in six axle units to stay within current limits, but both EMD and GE have shifted to a scheme where the traction alternator itself is split and operarted in series or parallel.

Modern practice, as of the mid 1908s, for DC drive is to eliminate transition by using GTOs or similar solid state device to limit motor currents when necessary at lower speeds (like below about 23 mph). Saves a lot on the necessary switchgear for the re-arranging the series/parallel connections. With AC-DC drive, there is no field shunting because all that's necessary is to raise the voltage output of the alternator.

"Transition" means "change". You go through transition to change the motor connections from series to parallel, thus the phrases "series transition" and "parallel transition" oxymorons. Same goes for calling the field shunting steps used in parallel as "transition" steps.