I think in this particular case, it's worth examining why we use roadtrains here in Australia because I believe the reason can translate directly to the Dark America scenario.
As I've mentioned in a few places
, Australia is a large continent with a small population. Rail links between major population or industrial centres are viable only because the service is more efficient and/or profitable for bulk loads. For example, in Western Australia, many of the passenger train services that ran from Perth city to rural towns where closed down in the 1980s. They use road coaches instead because there were not enough passengers per train to be economical for the service operator (and road vehicles are more flexible in that they can be easily redeployed to other towns as needed).
The same situation applies to freight but specifically in relation to the distance between towns here in Australia. It's just not economically viable to run a freight train over several hundred kilometres (and sometimes thousands of kilometres) to service maybe five or six towns because those individual towns have a population too small to need the large deliveries a freight train is capable of.
Most of those towns are dozens and in a few cases hundreds of kilometres apart from each other.
It's also important to mention that most inland towns don't even have a rail link because it's not economically worthwhile to maintain a railway track to those locations.
So the roadtrains operate in all those areas where a decent load of freight is required (but not large enough to justify a rail delivery) and all those areas where there is no rail link.
How does all that relate to Dark America?
Railways are much more labour intensive to maintain than roads (or tracks, some "roads" in Australia are dirt or gravel because it's too expensive to surface them with tar and maintain them). Rail tracks must be checked regularly and also, rail links need to be monitored to control the flow of traffic, particularly in areas where there is only a single track for any great distance. In the Dark America scenario, the railways are probable all controlled by corporations. They are not going to run a service that is not profitable, they are not going to maintain a section of rail if they can only make a profit from rail services every now and then.
I think we would see a lot of rail services to smaller towns and suchlike cancelled because it would not be profitable for the service operator - especially when you factor in how easy it is to stop a train simply by damaging the track (either by normal wear & tear, natural disasters or deliberate sabotage). In those cases, the truck is going to be the only good alternate for transport. Considering that double trailers are already in use in North America, it's not a big step to make it three trailers or four.
It is also worth noting that repeated use of roadtrains on a section of road can do quite a lot of damage to the road/track, sometimes causing a corrugated surface on dirt/gravel that makes travelling over them in lighter vehicles quite unpleasant because their suspension doesn't cope as well with all the dips in the track. Other damage includes breaking off chunks of tar from the edge/shoulder of the road and increasing the size of any potholes that are caused by weak road foundation.
It should also be mentioned that, just like a train or a large ship, you can't stop a roadtrain at speed in any sort of short distance.
