Most of the trucks nowadays can be replaced with trains. Let's focus on that first.
This it not an engineering issue, it's a political and organizational issue.
Because 90% of worldwide energy anything is currently non-renewable: https://en.wikipedia.org/wiki/World_energy_consumption
Power to gas is actually a technology employed productively to convert excess renewable energy into hydrogen etc.: https://en.wikipedia.org/wiki/Power-to-gas
I fully agree with your goal, though I do think this isn't a first this than that situation.
I don't agree. Global warming is an issue that should be approached with a multi-prong attack. And, if there is a future where hydrogen is clean and abundant, there's no point waiting for it to start working on making it useful.
Yes and no. Trains do not work well for distribution centers like walmart, krogers, target, etc... The last 1-100 miles will need trucks.
Long haul, yes, trains _can_ be used. But there are situations where that is not a feasible solution.