Ok, I finally got it sorted, and it makes sense, but I haven't seen anyone post exact steps to make it work:
1) have the grid mod enabled
2) put a power source (and battery if you want) in the grid for a cargo wagon.
3) put a roboport in the grid for the cargo wagon.
4) get into the cargo wagon. (this was the main part I was missing)
5) stop the train

Now things should be building from the cargo wagon inventory, and using the equipment in the wagon grid. I wanted to post incase others were struggling to get it to work correctly like I was.

Thanks for the tip, that was just the information I was looking for :-)
I'm starting a new "brave new world" scenario, and was looking for a tool to bot-build with a train.
And many tks to Optera for this mod ;-)

Steps 4 and 5 are incorrect.
4) put construction robots and items to use for construction in the wagon
5) optional enable logistic while moving so they work from driving vehicle

@chrinor2002 I've noticed that too, it seems you have to actually get into the cargo wagon once you've set it up before the bots inside will start working, which is more annoying now in 0.17 as you have to remove the locomotive first as it will teleport you to the locomotive if you try to get inside the cargo wagon...

it's not just cargo wagons though, other vehicles do it too

I think you need to tick the enable logistic while moving option on vehicle grid. I check the option and the bots on the rail car start to build following the blueprint

...which is more annoying now in 0.17 as you have to remove the locomotive first as it will teleport you to the locomotive if you try to get inside the cargo wagon...

@Walkman100, I'm fairly certain that the game doesn't automatically teleport you to the locomotive. It is possible to ride on a moving train from within a cargo wagon or even from within a fluid tanker wagon. Vehicle Wagon flatbeds do not allow a passenger, however, and I'm unsure if it is possible to ride within an arty wagon.

yes, I've since realized a mod is doing that, everything else in my comment is valid though

Ok. In coming back to read this notification, I'm realizing how old your comment was. It was late, and... well, you know how Factorio interferes with a person's attention span. :)

@Optera: I'd love for the ability to disable grids on vehicles. I love the idea of having a logistics buggy, but a grid on tanks makes them too overpowered for my tastes.