Depends what you want to use the battery for.
If it is only going to be a couple of times a day for depot shunting and a ‘limp’ mode for occasional use when the power is off, or some (very, very unlikely) problem with the tractors system between the pan / shoe and the battery, then an automotive spec Lithium ion battery pack of about 200kWh would be good enough to move a 4car emu for around 30 miles. That will weigh a bit more than a tonne or so, which is essentially negligible on a 4 car unit weighing 150tonnes+. As the full battery capacity would rarely be used, this would be good for 10-15 years or more, ie replacement at midlife refurbs.
If it is going to be used repetedly every day, then you want Lithium Titanate (LTO) batteries. About twice the price, and a little less battery capacity per unit of weight, but has the benefit of much less degradation, and thus many more ’charge cycles’ before needing to be replaced. It can also be charged more quickly. An LTO battery of 200kWh would be nearer 2 tonnes, again essentially negligible on a 4 car unit, and be about the size of a double fridge / freezer.
The ROG 93s have LTO batteries, albeit only 80kWh IIRC.
As
@zwk500 says, none of this is a problem. I fully expect that all future orders of electric rolling stock, and probably any diesel-electric stock too, have some form of traction battery on board.