This line was built by a predecessor of the Santa Fe in the late 1800s. It was part of the "passenger main line" (Second District) between Los Angeles and San Bernardino, via Pasadena, until the late 1960s. At that time, I-210 was built a few blocks north of the right-of-way through Pasadena, and the main line relocated to the median of the I-210 freeway.
The original main line left behind by the 1960s freeway relocation became a spur, joining the "new" mainline at a flyunder junction in the freeway median (photo below). This spur served lumber yards and small businesses in East Pasadena and Lamanda Park, but it was abandoned in the 1980s.
The Santa Fe routed most of its freight through Fullerton; the line through Pasadena was used for passenger trains and a few piggyback trains and locals. The portion between Los Angeles and Pasadena is scenic and winding, but not really suitable for fast freight. Amtrak ran both the Southwest Chief and Desert Wind over this line, but relocated the Desert Wind to the Fullerton Line in 1986. When Santa Fe abandoned the route between Los Angeles and Arcadia in 1994, Amtrak rerouted the Southwest Chief through Fullerton also. Except for a short re-routed section near Union Station and Chinatown, this former Santa Fe line re-opened as the MTA Gold Line Light Rail service in July 2003. The Arroyo Seco bridge, midway between Los Angeles and Pasadena, was retained and double-tracked.
See also the The Pasadena Branch.