Is your feature request related to a problem? Please describe.
Currently there are only FE/RF enegry loaders and unloaders and one flux cart that can't be used with just GTCE installed. (There are addons that can convert energy but doesn't feel quite right and is super impractical).

Describe the solution you'd like
Energy (un)loader that works with GTCE power, preferably versions for all 4 electric tiers so they can respect voltage tiers too. Going hand in hand there would be battery buffer carts that can be filled/emptied. (maybe a config for the internal buffer of the electric locomotive would also help adjusting balance).
I am not sure how this would be balanced in detail when it comes to hopper interactions for batteries and so on, the drag for these carts should probably be high enough so they cannot run without electricity to avoid lossless energy transfer.

Hmm, I need to look into gtce code. this will most likely be pr'd to gtce instead of added to railcraft directly.