I'm having trouble figuring out how to get the wagon to take from the accumulators. How do I do this?

There's two special circuit signals under the far-right tab when you're looking at combinator settings - the trains check for these signals when they arrive at stations to set their charge or dischare priority. To get them to respond, send one of the special signals on a wire hooked to the train station - the easiest way to do this is with constant combinators sending the special signals wired directly to the train station.

Thank you for the reply!!

Integration with this mod: https://mods.factorio.com/mod/Advanced-Electric-Revamped-v16 could fix the problem of low wagon capacity.