I suspect there's a maintenance depot somewhere near the bottom station, which is likely to have a charger. It's just that's not the routine use case.
Or you keep a diesel generator on the locomotive itself, making it a sort of hybrid.
Diesel locomotives are already hybrids (pretty much all diesels built since the mid-20th century are diesel-electric). I imagine that a generator large enough to charge up a locomotive would add too much extra weight to the design, however I could see a modified diesel locomotive being utilized to both push a dead electric train and to at least partially recharge that train's batteries during the journey.
Diesel-electric drivetrains are not "hybrids" in the sense that they have an energy storage device. The same type of drivetrain was used in submarines and tanks during WWII, no one calls those a hybrid.
You keep a diesel locomotive at hand. They probably need one to move along the track to do repairs. Not sure if it's got enough HPs to move ore back, but anyway, why should they want to put ore back into the mine? Sending equipment yes, but it's probably light enough to travel on the return train.