is it possible to make a waggon for a train? like they used to have for track repairs. would be nice to have a train repairing stuff at stations and along the ride.

why not make a "Repair Tower" that i can build an feed with repairpacks?

It would be hard to make it working when the vehicle/train is running. Due to game mechanics, if a robot would run outside the construction area then it will hang in place waiting to be covered by some roboport once again. So basically a robot would fly out of roboport in order to repair something and then will stay there, because the train is already far away.

So in general, roboport could work well when not moving. Regarding rail maintenance:
https://mods.factorio.com/mods/Choumiko/FARL

There is a mod called "Wagons" that was revived once by 5dim it offered roboport wagons and mounted turrets, perhaps have a look at it https://forums.factorio.com/viewtopic.php?t=3926
it probably offers some insight on how to realize it, if you wanted to revive it, i'd be glad to help! I can only make easy luascripts and judging your mods your coding skills are far above mine, so eh I could help fixing old and new arguments, missing type definitions (if i can make them out!) and general copy pasta stuff

it was a great mod and 5dim ported it to 0.12 but i can't find the download anywhere

also isn't any roboport kinda a repair tower?

robots will go to the nearest roboport if left in the wild, if they run out of power it might take a bit longer

I also like the idea of offering alternatives to the powerarmors which are kinda needed, especially with any mod that increases enemy difficulty

This works as a portable Roboport, it has a slightly smaller network size than a standard Roboport but should still be useful when setting up new bases or using as a portable repair system. You have to be stopped for the Roboport to be active.

So author was dealing with the same problem as I described above. This roboport may work well only when the vehicle/train is stopped.

It's not a problem to introduce a roboport wagon having a working code for roboport tank, the logic behind is quite similiar. I don't know if it would make much sense.

However I will consider this if I would have some free time.

If you'd put it on github i'd be more than glad to help, I probably won't be much help with most things concerning control.lua but i'm at least able to do what i did with assembly zero

having to stop is a small problem, there is also an interesting mod that turns rails into triggers that turn inventory on them in respective logistics containers
that way one could make a parking lot for tanks I think