Yeah you could go fully electric for a train. I don't know if its really worth it or not though. Cost of diesel engine vs batteries I suppose, and if the batteries cost more, how much for that on every train as opposed to just electrifying all the rails.

Current diesel electric is pretty good, adding a short distance battery might help some of the routes that branch off a little from main lines.

Although some train operators are still running diesel only even though the rails have overhead power.