I think I disagree in terms of battery units. They work; the technology is sufficiently developed for that, in the right service (as I noted, not climbing long grades, but in places where variable short-term assist is needed and on-road recharging by just dragging a little is feasible). You'll never see a train run exclusively by a battery locomotive. But you will see locomotive consists with a battery unit for "hybrid" boost and fuel economy improvement/net emission reduction. It'll be a niche market, though, and will be driven more by regulation and incentives than by economics until fuel prices go higher, battery prices come down a lot more, or both.

One place I could see them being useful is in commuter and regional trains. Instead of a Cabbage, a battery unit could provide cab car service at one end with a diesel at the other. Would allow longer trains and faster acceleration, and the frequent start/stop of commuter service is an ideal operating mode for a hybrid/battery system due to regen during braking. You can see that with on-road hybrids: maximum fuel economy is in the City test mode, not Highway (though my Prius in real life works the other way around like a normal car).

It would be tons easier to manage than a true hybrid locomotive (if one could even be done cost-effectively) - if something goes wrong, just shut it down and drop it at a suitable location like any other unit, and continue (more slowly) using the remaining diesel power. In all, it seems like a good gimmick, as long as the railroad gets somebody else (aka taxpayers) to pay for it.