You mean like this US section
I think it was meant to be handled individually by each country.
What should likely be clearly stated everywhere else is that it should NOT be connected to the current road grid at all.
The main question is, if it's not drivable, what's the use of setting turn restrictions ?
Would anyone riding in a train want instructions to turn left ? It's not likely that they would be routed on there by the routing engine in the first place.