Of all the "established" standard products out there, IMO diesel-electric locomotives are one of the most logical to transition to a hybrid platform....
1) Some of the key design concerns/limitations with drivetrain batteries in other platforms almost go away (bulk, weight)...... opening the door to selecting large amounts of battery storage, and possibly chemistries that aren't as practical in other applications.
2) The drivetrain is already in place, and so is/was a lot of the major electrical power switching........ IIRC, "Old school" diesel-electric Locos had huge power resistors and fans in their roofs, to deal with dissipating the back-emf created from the electric braking function.
So yes, the added drivetrain storage batteries would have to be appropriately selected and charge controlled, but compared to standard buses, trucks etc, DE locomotives are 80%+ a hybrid platform already......
Rgds, D.