And signals, and don't forget about the likely need for bridge replacements. Siding spacing likely to be longer than 12 miles. More likely 15 to 20 miles. Conversely, given the open country, track per mile cost would be far less than a contractor's price for the usual transit or commuter line track, maybe even by half.Rebuilding the line is going to be major work, might as well start fresh with concrete ties, CWR and 15,000ft passing sidings every 12mi.
Given UP won't even spend the money to finish double tracking the sunset route I'm doubtful they'd rebuild a large section of main track for 1 Z train a day.