As of now all GNSS constellation are somewhat spoofable, especially if you only want to drift a clock a few milliseconds late... And a lot of timeservers are still GPS only so the attack is pretty easy to perform.
The private line idea is maybe doable (but which protocol? NTP doesn't offer the needed precision), but as all substations would need one, my guess is that eventually money would be an object...
The private line idea is maybe doable (but which protocol? NTP doesn't offer the needed precision), but as all substations would need one, my guess is that eventually money would be an object...