Excess from wind and solar are better handled by mechanical energy storage. The two examples I'm aware of are magnetic flywheels and Pumped-storage hydroelectricity, each working on a different scale. Thermal energy storage should also play a role in specific applications. A flywheel (or a battery thereof) in every basement would help smooth out any electrical failures, potentially holding things over for hours at a time, enabling extremely resilient energy infrastructure. Pumped-storage hydroelectricity is extremely cheap and efficient for what it does; I'm not aware of any options that are better, so long as you're cool with a lack of mobility and want something very large-scale.

I'm not super familiar with the numbers when it comes to trains specifically, but I'm definitely skeptical. As far as I'm aware, one of the reasons it works for rocket fuel is because that application immediately and completely uses the fuel, and so leaks are a relatively small problem. Another is that hydrogen's weight scales very favorably (it's the lightest fuel we suspect to be physically possible), and rocket efficiency is highly dependent on fuel density per joule. Trains can accommodate very high loads, need to hold energy for long periods, and don't get a huge benefit from reducing the mass of the energy storage system as they go; it's entirely possible that flywheels beat out hydrogen (Note: the main disadvantage of flywheels is that they are heavy, and so are not generally optimal for vehicles) and I have a hard time imagining hydrogen beating out rechargeable batteries, but again I'm not familiar with the numbers and so cannot say for certain.

you don't know what helpful discovery might be found

That's entirely fair; experiments are never worthless. However, our current understanding still makes hydrogen bikes extremely unlikely to be effective.