Surely the demand would only be for a battery pack with enough capacity to get an electric train through a bulk freight loader, or through the sidings around an intermodal terminal without needing a shunter.
It's got to be more efficient (operational efficiency, not energy efficiency) to have a battery pack than attaching a shunter to pull an electric train through a freight loading facility. The big question is of course whether a battery pack would be more cost efficient than a small diesel genset that would be off-the-shelf technology.
With bulk freight at least, distributed power provides a simple solution to the issue of what happens going through the loader without needing any extra power source (battery pack, genset or coupling up a shunter). Multiple locomotives split in different parts of the consist (controlled via Locotrol or over the fixed data connections if ECP is in use) should allow propulsion to be handled by the other locomotive/s during the part where each one goes underneath the loader.
The Australian freight company Pacific National operates electric coal trains with 120 hoppers that are marshalled as loco 1 & wagons 1-40, loco 2 & wagons 41-80, loco 3 & wagons 81-120. When passing through the loader, each loco lowers its pantograph (on the 'slave' locos this is controlled from the lead 'master' locomotive) while the propulsion is handled by the other locos in the consist. Each of the loco+40 part-consists also rotates through the maintenance depot as a fixed formation, so even getting it right into the heart of the depot is easy when the other parts of the consist can push/pull it.
