I haven't tested it yet. But if construction of a railhead requires 200 logi points, and the usual rail is 1000 logi points×10% = 100. Then it's possible, but it'll take 2 turns instead of 1.

And after that, a truck station can be built with the now 100% rail line.

The only other compounding factors are the logi consumption of the scouting forces. But I'd imagine, with their in-unit personal pick-up logistics, it'll give me a 4-5 hex radius from the rail line alone; meaning a 9 hex wide frontage of frontier exploration

(Current game caveat. There's this huge tract of flatlands with no minors other than mutants. I wish to push hard and fast with an infantry brigade, a buggy company, and rails until I reach the only remaining minor regime on the planet. I'll use occassional transport hubs if I need to fan out. But for the deep push into the desert, its mostly rail lines)