This discussion comes up every few months. Go back to a thread in mid 2008 titled The Sun Never Sets in Phoenix.
I said this at that time:
Other than the date of change, which I really don't know the rest is somewhat less than accurate, which seems to be the norm for Wiki.
The southern line, which UP calls the Gila subdivision was the original Southern Paciifc mainline. The loop through Phoenix was built much later, in the 1920's I think, and was always at best a secondary main and built primarily so that through passenger trains could serve Phoenix. It ran from Wellton at milepost 770.7 (zero at San Francisco) to Picacho. At Picacho the milepost is 979.7 on the Phoenix line but only 936.7 via the main through Maricopa. Thus, the route is 166.0 miles on the main and 209.0 miles through Phoenix, an additional distance of 43.0 miles. Phoenix yard is at milepost 907.0, so this 209 mile length represents 137.3 miles wst of Phoenix with almost nothing there and 72.7 miles west of Phoenix through Tempe and a few other places.
The Phoenix line was all jointed rail with an automatic block signal system with semaphores. Generally the passenger train speed limit was 60 mph, although some parts east of Phoenix allowed 70 mph. Pictures I have seen of the line west show the rail as being 113 lb, a section unique to the SP and long obsolete and out of production. By the early 90's this line was needing major rail relay, major tie replacement, and signal system upgrades. The decision was make to concetrate on the 70 miles east of Phoenix rather than attempt to keep the entire 209 miles up for 60 mph service.
To the best of my knowledge the track is all still in place, but not all in service, and the signal system is likewise all in place, but likely not functional for most or the line west.
With the same weasel words, here is the current status of the line, going from west to east.
mp 770.7 to 802.8: "Roll Industrial Lead" 20 mph - restricted speed
mp 802.8 to 854.0: Out of service
mp 854.0 to 904.8: 25 mph - may have slow orders to less
mp 904.8 to 905.6: 20 mph
mp 905.6 to 906.7: 15 mph
mp 906.7 to 907.9: 20 mph
mp 907.9 to 913.6: 25 mph
mp 913.6 to 916.5: 20 mph
mp 916.5 to 920.8: 40 mph
mp 920.8 to 922.0: 25 mph
mp 922.0 to 924.2: 40 mph
mp 924.2 to 957.0: 60 mph
mp 957.0 to 963.5: 40 mph
mp 963.5 to 975.6: 60 mph
mp 975.6 to 979.3: 45 mph
mp 979.3 to 979.7: 25 mph
Wild guess: about $200 to $300 million to get the Phoenix line west to the point that it could carry the Sunset at 60 mph or 79 mph. (The difference in cost for getting it in condition for 60 mph and for 79 mph would be next to nothing.) There is no way that it makes sense for Amtrak or the state of Arizona to spend that kind of money for a three times a week train, and for sure it makes no sense for UP to spend it. The additional 43 miles and going through a major urban area is why it also make no sense for UP to increase the Sunset Route capacity by upgrading the Phoenix line for part of a directional running system with the Gila line instead of adding a second main along a line through a mostly unpopulated desert. Now, if it was decided to operate multiple passenger trains per day between Arizona and Los Angeles, that could change the picture.