The Expo line ROW is entirely on the old the PE "air line", which was the original route of Los Angeles and Independence narrow gauge to be built from Santa Monica to the Owens Lake, via Cajon Pass, by the Silver Barons. This line was to compete with the SP coming over from Bakersfield to tap into the Panamint/Cerro Gordo silver trade, which came on the original 20 mule team trains run by Remi Nadeau to LA and then on the Los Angeles and San Pedro RR, to the distant "Port" of Wilmington.

The LA&I never made it past LA, but grading and construction had extended all the way to Cajon, including initial excavation of a tunnel under Cajon summit, but it was never finished, as SP had already reached Mojave taking the Cerro Gordo trade north to SF. The line was bought by the SP to solidify its monopoly on Sou Cal, and after several iterations of SP ownership, including years of direct standard gauge LA to SM SP service, from old LA Arcade and 4th Street stations, was consolidated into the PE in the late 1900's.

The LA&I was to have eventually connected to a proposed Carson and Colorado RR which was to be built down from Mound House, on the Virginia and Truckee, to Owens Lake. The C&C started construction five years later in 1880, and which reached Keeler in 1883, as the silver boom crashed..... Completion of the C&C prompted some wags to comment that it had either been built 300 miles too long, or 300 years too soon....to Keeler, still in the middle of nowhere, today. This narrow gauge line was sold to SP around 1900, just prior to the Tonopah mining boom, and became the SP narrow gauge from Mina to Keeler, with a connection to the SP Jawbone Branch, built a bit later to support construction of the LA Aqueduct.

Expo is an excellent example of LA RR history coming full circle.

That is also a pretty good (ambitious) running time, given the number of stops, and very competitive at rush hour with the 20mph average speed to Santa Monica on the I-10..... Lots of private ROW (a legacy of the route of the LA&I) and some well placed grade separations allow for decent speeds on some of those segments.