Yes, I was nice and quick thanks to remarkably clear traffic conditions on the 20th-century 6-lane M42 and M40.
Ooh, you do enjoy the difficult questions!

The rules are quite clear (IMO) about the movement allowance on Rds only being for march moves. That means that train (who don't have a tactical move allowance in DGo) slow to what is effectively a halt in the presence of enemy elements. Think of this as your baggage/artillery train needing some guards sufficient to see off enemy troops. Alternatively, don't put your train in a Rd in DGo if there are enemy nearby!!
Troops using the Rd in the BUA move quicker when marching, just as any troops move quicker when marching. Presence of nearby enemy restricts that. If you want some sort of ad hoc rationalisation to try to justify what is really a rules-artifact, perhaps the local civilians are wailing and renting their robes, pleading with your troops and their commanders to save them from the vile enemy hordes appearing just outside the walls and this is slowing down the military's response-times.

As for the deployment issues, p.22 "No element can be deployed in terrain it could not enter." Since these troops could have entered the terrain on the Rd, they can be deployed there. Easy really.
Tim