Wikipedia
The San Diegan was one of the named passenger trains of the Atchison, Topeka and Santa Fe Railway, and a “ workhorse” of the railroad. Its 126-mile (203-kilometer) route ran from Los Angeles, California south to San Diego. It was assigned train Nos. 70–79 (Nos. 80–83 were added in 1952 when RDCs began operating on the line).
The Los Angeles-San Diego corridor (popularly known as the “ Surf Line” — officially, the Fourth District of the Los Angeles Division) was to the Santa Fe as the New York– Philadelphia corridor was to the Pennsylvania Railroad. Daily traffic could reach a density of ten trains (each way) during the summer months. The first San Diegan ran on March 27, 1938 as one set of equipment making two round trips a day.
A second trainset delivered in 1941 made possible four streamlined trains each way. A set of heavyweight equipment made a fifth trip in each direction. During and after the Second World War, furlough business from San Diego's military bases necessitated extra (albeit heavyweight) sections of San Diegans, and racetrack specials during horseracing season at Del Mar added to passenger train miles.
Amtrak continued to operate the San Diegan when it took over operation of the nation's passenger service on May 1, 1971, and it retired the name on June 1, 2000. Today, the route of the San Diegan (the second busiest passenger rail line in the United States) is served by Amtrak's Pacific Surfliner.