Could it be good for a long-haul truck?

or maybe a locomotive, which is less sensitive to weight and cargo capacity?

It kinda depends to what power density it gets. Trading say 30% range for ability to refill the truck within minutes might just make enough sense to work. But if, say, electric truck gets enough range to run the full 9 hours (max allowed driving time per day in EU for truck drivers), then it doesn't matter as trucker can do "100% of their work", get to stop and leave it to charge for rest of the day.

For locomotives ? I'd imagine the simplest solution would be to have "battery wagon" ; get on the station, replace the battery wagon, and be on your merry way while it is recharging in the station waiting for next train to pick it up, at least for the long haul stuff. But capacity density still matters, if flow batteries are being close and cheaper then it makes sense, but if you need to pay slighty more per KWh but get 2x the capactity... thats 2x the range and less maintenance.

Or, you know, just electric rail. It already works fine in many places. I wonder if making some hybrid solution with electric rails also being covered by solar panels (basically so same maintenance crew can manage both) would make sense

So it all depends to what level of density it gets. If it is same capacity per kg but lower power there are still many places it can be used, if it is much lower it stops being sensible for moving stuff. And the price would need to be significantly below the normal batteries for anyone to even bother.

Trucks are weight limited with LFP, costs already favour electric even with NMC but the tradeoff is payload.

Trains maybe, but surely the money is better spent on overhead wire? Even if tunnels/bridges/etc. can't take it you're better with just the power-buffer battery and no flow battery.

Isn't the costs and recurring costs of railroad infrastructure orders of magnitude larger?

Full electrification is expensive, but overhead wires where it doesn't require rebuilding anything isn't.

And rail is far cheaper to operate once built per unit capacity than trucking. If you don't inflate it with endless feasibility studies and contracts that get paid regardless of output the initial building costs are about double a 4 lane highway, but it carries a lot more and maintenance is lower.