They are not detectable anyway - if you put a nuclear warhead in a truck, there is no way to detect it from a plane or a satellite,

In fact you probably could put a nuclear bomb inside a lead-lined shipping container and send it from China to US, across all US railways, and back across the world on another ship, and probably no-one would ever know.

Also why would Russia move nukes by land through chernobil, when they have bombers and complete control of the airspace?