First of all:
I got this idea from this video https://www.youtube.com/watch?v=pDLIkfSLTmU
I don't have the time or skill to redesign the exact outpost used so here's a quick blueprint you can throw down if you want to have a go at it. Get a hideous ammo loader station for a smaller version of an artillery train here: https://factorioprints.com/view/-L6o-pAtfRES-hD6YI7S
concept inventor : Michael Lundgren https://www.youtube.com/channel/UCfvDu_ejotNo5d-Oku0ssXw
(as far as I know)