[4] A proposed four-mile (6.4 km) extension of the line from Glendora to Lone Hill was denied by the Railroad Commission of the State of California in March 1918, citing wartime conditions.

The last cars ran over the line in the early hours of September 30, 1951 — the final passenger run to operate over PE's Northern District.

A mile or so further north, the four tracks crossed over Mission Road on a concrete viaduct, and then continued northwesterly on private way between the dual roadways of Huntington Drive.

It continued northeasterly still between the dual roadways of Huntington Drive, passed Oak Knoll Avenue, (where the Oak Knoll Line proceeded north into Pasadena), and in a mile or so passed what is now known as Sierra Madre Boulevard (where the local Sierra Madre Line turned north) in San Marino.

Proceeding east, the Monrovia–Glendora Line passed southerly of Santa Anita Racetrack and continued northeasterly on private way to St. Joseph Street in Arcadia.

The line crossed the river on a double-track concrete arch viaduct, which was reduced to single-track following a 1938 flood.