Which we won't have to do to just repair the existing railroads. My point is if they can build railroads with almost no industry and no `international supply chains` then we can get the already working rail system started again, no?

PS: even in more humane phases of the Soviet Union I doubt very much that self-sufficiency had worked. For instance large infrastructure projects, like the Baikal-Amur railway or the Drushba pipeline were built with massive support from all Eastern Bloc countries, not just financially but by sending entire construction teams over (I remember well the propaganda campaigns for this in Eastern Germany).

The VAZ factories were built by the Italians, and the early Shiguli/Lada car models were phased out Fiat models.

The MiG-15 prototypes famously flew with English jet engines.

The Tu-95 turboprop engines have been designed by Germans.

Etc etc... my point being that self-sufficiency doesn't work in the modern world (unless the people are content with living like medieval peasants).