JR East has been running the EV-E801 BEMU on the Oga line (Akita prefecture) for the better part of a decade. It can run about 110kph under wires and 85kph on battery with a battery size of 360kWh, arranged in a 2-car set.
It’s a proven unit built for 1,067mm gauge and doesn’t seem to be over-engineered for the task at hand, if Gatton is the interim objective terminus.
Potential media release here:
I thought it was impossible to electrify through the tunnels without killing the heritage element? Or maybe they just need to be dug down so far it’s more cost effective to reroute?
You could do it, but you’d have to dig down far enough that you don’t rely on the tunnel structure to hold the overhead lines. Victoria tunnel is brick lined and of 1860s construction - no way you could bolt overhead line equipment into it so it would have to stand independently.