There was a reason why the BoS had to use airships, just to get to the MW from Cali. It was still retarded but meh, spinoff.
I mean Beth could have used some tanker MCGuffin like 2, and took the sea route. Thats even more believable.
Beth kicked crazy up to 11 and simply said they MARCHED to DC. As if bridges and parts of the earth hadn't been changed by the nukes. You have collapsed buildings, rockslides, mudslides, missing roads, etc.
And yes, radioactive twisters/storms, deathclaws, the retreating SM army, the early legion raiders, etc.
I forget if Beth treats Tactics as canon or not but even with the peace of the MW chapter, they do not control territory all the way to DC.
Like I said, the Enclave could have brought/started their own operations. Or someone else invented the same damned thing. I am not being inflexible here but the reasons have to make sense besides, Beth said so.