This is a former Pacific Electric line, and originated at the Southern Pacific Mainline at Control Point "Bassett" (located in City Of Industry, in the vicinity of the Temple Avenue and Valley Boulevard grade crossing). The alignment followed a generally northeast heading, passing through Industry, Baldwin Park, Covina, San Dimas, La Verne, Pomona, Claremont, Montclair, Upland, Rancho Cucamonga, Fontana, Rialto, and into San Bernardino, where it joined up with local San Bernadino lines, as well as the The Rialto Branch line and the The Redlands Loop.
Between Industry and Claremont, the line ran on an exclusive right-of-way. Right around White Avenue in Pomona, it joined up with the Santa Fe Second District (a.k.a. Pasadena Subdivision) and ran parallel to the south of that line for a couple of miles. Just inside the border of Claremont at a point known today as CP "Cambridge", it crossed over the Santa Fe line at grade and ran parallel to that line, but to the north for a few more miles before heading off on a northeasterly tangent away towards San Bernardino.
It was built sometime in the early 1910s and remained in service with PE until they abandoned the line. It was taken over by Southern Pacific in the 1950s and was used as a "local", serving a number of industries. By the late 1980s, this line was in a very poor and dilapidated condition. Track speed was limited to just 10 miles per hour. By then, movements were reduced to once a week, serving a few remaining businesses in San Dimas, Covina, etc.
Today, the line exists in four incarnations. At the start of the 1990s, when Metrolink was still in the planning stages, it was decided that the line west of CP "Cambridge" and the Santa Fe line the rest of the way to San Bernardino was to be used for Metrolink's very popular Los Angeles-to-San Bernardino line. In early 1991, all of the original SP/PE track was removed and the new high speed rail for Metrolink was laid. CP "Cambridge" was redesigned, and the Santa Fe Pasadena Subdivision became a spur or branch line. The rest of the line between Cambridge and Rialto was abandoned and removed in stages throughout the 1990s.
From San Bernardino, extending west about 4 miles to Rialto, a short section of the original line remains in place and active with Union Pacific. The line ends at a dead end just west of Lilac Avenue in Rialto. From Rialto to Upland, most of the ROW is still intact, including an attractive bridge that crosses over Foothill Blvd in Rancho Cucamonga. But it is truly abandoned with no track of any kind left. This section of ROW is often used for bicycling, jogging, etc. Moving west, there is about a three or so mile stretch of the line in Upland and Montclair that has been rehabilitated into a dedicated pedestrian and bicycling trail, complete with exclusive pavement and traffic signals at the former grade crossings. Finally, the ROW that ran parallel north of the former Santa Fe line is currently unused. Current planning calls for this ROW to eventually be rebuilt with the LACMTA Gold Line Extension. But as of this writing (2006), that project is definitely a long way off yet, and there are no guarantees that it will ever happen.
Thanks to Matt for contributing information about this route.