Can they be used on units in the Railroad box?
I suppose you mean the Free Soviet upgrades of rule 7.2.
Rule 7.0 states:
A. The Flip Step
Important: Only units that can trace an Overland Supply
Path (6.1) may be Improved (7.1) or Upgraded (7.2).
Since they are not on the map they cannot trace an Overland Supply Part (6.1), thus cannot be upgraded.