In theory, you could put lines next to the SP, like the UP did in Sou Cal., or go via some of the "mountain" branches in AZ and NM, to make various connections. But, there was no reason, they weren't needed, and couldn't generate enough local or long distance traffic to bear the cost of construction and/or operation. It is surprising the level of rail system that existed in the far west by 1910. How many rail connections do you need to Tonopah Nevada, after all?

As early as the "Erie wars", and the great "panic" of 1893 (a massive crash in rail stocks..), big money folks realized that in many markets, competing long distance railroads were overbuilt, and many, like the Erie, had not been making any actual money for stockholders (nor would it until WWII).

Financier's like Harriman and JP Morgan had realized that properly capitalizing the best networks, turning them into modern rail systems, and making them efficient was a better return on railroad investment. In addition, increased tariff regulation of service to the RR's customers, forced investments in RR operating efficiencies, which made more sense, than expansion that no one was willing to finance anyhow, after 1910. Roads underway were mostly completed, like the WP and LA&SL, but there was not much more rail system expansion after 1910.

Choo-choo's is an expensive business.... It was, even back then... But, still cheaper than feeding mules and horses....