Single direction signaling (for double track ABS) using track circuits is fairly simple. You install a track battery/feed and a relay towards the direction of the approaching train. If there is a train in the block, the entrance signal for the block is red. At a red signal, you reverse the polarity of the battery feed, and assuming the previous block is clear the entrance signal will be yellow. The pole line (if used) was just needed for AC power (if available), dispatcher's phone and other communications.

Modern signal systems use the rail themselves to transmit track block indications to the next signal. So if you are completely replacing the signal system anyway for PTC, I don't think the cost to upgrade to dual direction and/or CTC is that much more, especially considering the potential benefits. After that last big Amtrak mess where the signal system was suspended, there will likely be new restrictions to run against the current of traffic, basically dark territory.

As few trains as are left on the line, with a little planning they can get trains around a derailment or work window a whole lot easier. Does anyone know if they have changed the old manual detour crossovers (Elko and Carlin come to mind) into Control Points?