Will the fac be limited to only trains? Will you be able to "fit" smaller bots and vehicles into them?
This fac could have purly trains or trains along with semi-structural units that can mount or transport multiple units, or even create units. Or both. It depends how far you want to go.
Lauri has a interesting idea, trains without tracks. Maybe...something like a high-HP unit that can only move around whil powered somehow, while also being upgradable VIA new train cars to hold a grid or produce certain units. A cross between the spec-ops engie, a reef, and a power plant.
I have no clue how balanced in-action this would be though, i'm just throwing ideas at you.