The fixed cost and time consumption of electrification is quite high.
I'm not sure about hydrogen specifically, but just as steam trains had a coal+water "tender" behind the engine, it seems not unreasonable to have a "battery" car on a train. Whether that's a tank of H2 or just a boxcar full of 18650 cells (anyone care to do the maths on that?)
Which has the comment: At the same speed, same load (GCW) and on level ground, any steel wheeled railway vehicle is 24.6 times more energy efficient than any large rubber tire road vehicle, regardless of the type of power source.
My hand wavy argument would be, to go 300 miles a passenger car needs 33% (guess) of it's weight to be batteries. But a truck is probably 2-2.5 times more efficient. So would only need 33% / 2.25 (split the middle between 2 and 2.5) or 15%.
If a 4000 ton train is 25 times more effecient than a truck as started above then. 15%/25 is 0.6%.
So the battery for a 4000 ton train would weigh 4000 * 0.6% -> 24 tons.
Note that the majority of hydrogen trains currently under development are all multiple units, and don't have dedicated locomotives. In the UK at least, track access charges push things towards distributed traction in part because of the axle loadings of any dedicated locomotive (or battery car). You see this to varying degrees across countries where speed is relevant.
To take as an example: each British Class 802 motor car has a 1550 litre fuel tank. If we (slightly optimistically) assume the diesel engine is 50% efficient, and diesel fuel to have 10.8kWh/L energy density, then that's 8370kWh of energy produced. The Tesla 3 battery pack manages 160Wh/kg, so to carry 8370kWh the battery pack would be 52 tonnes. That's more than each car currently weighs in total. And a nine-car train has five such fuel tanks and engines.
I'm not sure about hydrogen specifically, but just as steam trains had a coal+water "tender" behind the engine, it seems not unreasonable to have a "battery" car on a train. Whether that's a tank of H2 or just a boxcar full of 18650 cells (anyone care to do the maths on that?)