OK, there are several what is where discussions bouncing around here, some of which have deficiencies in geography and railroads. Here is the "what is where" concerning the east-west railroads across southern Arizona, or more specifically between Yuma and Tucson. My source of information is primarily a Union Pacific employee timetable form October 2007. At that time all their employee timetables were on line and publically accessible. That is no longer true. That is to say, what is here is from sources that are or were publically accessible, and not from any inside knowledge.
Mileposting on this railroad increases from west to east to as faar east as El Paso, beyond which the mileposts are increasing in the east west direction.
The Main line between Yuma and Tucson, through Maricopa, bypassing Phoenix:
milepost Location/town
732.4 Yuma
770.7 Wellton (connection to west end of Phoenix Line)
897.7 Maricopa (may be +/- about a mile)
936.7 Picacho (connection to east end of Phoenix Line)
984.6 Tucson Amtrak (maybe +/- about 0.2 mile)
1295.9 El Paso Amtrak (maybe +/- about 0.2 mile)
note that the distance between Wellton and Picacho by the main line is 165.9 miles
Speed Limit between Yuma and Tucson is 79P/65F eastbound and 79P/70F westbound. I have no idea why the difference.
The Line through Phoenix:
770.7 Wellton (begin Phoenix Line)
906.0 Phoenix
979.7 Picacho (end Phoenix Line)
Note that this line is 209.0 miles between points where it connects with the main line. Therefore, it is 43 miles longer than the main line.
The condition of the line as of 2007 was as follows:
mile 770.7 to 802.8, 32.1 miles: "Roll Industrial Lead" speed limit 20 mph.
mile 802.8 to 854.0, 51.2 miles: Out of service
mile 854.0 to 904.8, 50.8 miles: 25 mph (That would mean FRA class 2 track conditions.)
mile 904.8 to 907.9, 3.1 miles: part at 20 mph, part at 15 mph.
mile 907.9 to 913.6, 5.7 miles: 25 mph
mile 913.6 to 916.5, 2.9 miles: 20 mph
beyond here in the remainin 63 miles to Picacho, even though the listed maximum speed is 60 mph, about half of it is 40 mph or less.
To be able to return the Sunset to this line would require major work which is not justifiable for a train tht only runs 3 daus per week.
the Union Pacific is in the precess of double tracking the full length of the Sunset Route between Los Angeles and El Paso. as of the 2007 employee timetable, the entire Tucson to El Paso distance ahs two main tracks. there were several sections west of Tucson in place then, adn there has been quite a bit more constructed since, but where exactly, I do not know.
Yes, this double tracking is helping the operation of the Sunset. Firstly, it is doing much better at keeping schedule. Second, with the last Amtrak timetable issue, it was speeded up from its pace of the last few years.