We can consider it a relatively straightforward upgrade to the system though. Definitely more expensive than upgrading individual trainsets to h2 or lithium, and nowhere near as quick... But it could be staged, or just the mainlines.
Imagine mainlines get electrified so EV or h2 trains use none of their onboard energy, until they start getting onto the unelectrified branches.
That's exactly what New Zealand is doing with battery electric trains, the plan is for them to run on batteries once they get beyond the overhead line network, to service areas where it's not worthwhile to have overhead lines.
Not with hydrogen trains though, that's a dead end technology.
Again, not a fan of how the h2 is sourced, how limited its surrounding infra is, etc etc. Just saying that at least for trains, it makes more sense than for a car.
I do like that it's approximately an EV that isn't lithium constrained, but plenty of serious downsides as well, see: intra-atomic escape.