By the late 1930s and early 1940s, 4-6-2 Pacific type locomotives were assigned to the heavier peak- hour trains. Above, 2479 at an undisclosed location. In 1958 2479 was donated to Santa Clara County and placed on display at the fairgrounds. Today the Pacific is under restoration. (Rattenne Collection)
 

Former SP 2472 races past the historic Milbrae depot during a break-in run in 1991. As additional 4-8-2s arrived on SP, more 4-6-2s were assigned to commute service (GGRM)
 

This  is about as rare a steam incident as you could get on the latter-day SP. SP 4488 is at Third and Townsend in the late afternoon of a 1956 summer day. Northern 4488 is ex-SSW 813, built by the SSW shops at Pine Bluff in 1937. Retired 11-13-58, and scrapped by Luria Bros., the 4488 ran only a few weeks in commute service. (Rattenne Collection)
 

  In 1860 the San Francisco and San Jose Rail Road was incorporated. Financing for a railroad between the two cities came from three counties --San Francisco, San Mateo and Santa Clara. In May 1861 ground was broken at San Francisquito Creek between Palo Alto and Menlo Park and five construction camps were set up to start work. In October 1863 regular service between San Francisco and Mayfield (now the California Avenue section of Palo Alto) began. The trip took two hours. At Mayfield, passengers boarded a stagecoach to get to San Jose. 

On January 16, 1864 the line was completed to San Jose.. More than 2,000 people attended the ceremony marking completion of the line. Within a short time, two trains operated each weekday between San Francisco and San Jose. In 1870 the SF&SJ Railroad was absorbed into the Southern Pacific, controlled by the "Big Four" -Leland Stanford, Collis P. Huntington, Charles Crocker and Mark Hopkins. In 1907 the Bayshore Cutoff, a 10-mile line which bypassed the original route through San Bruno and the present Daly City was completed. 

The opening of the railroad down the Peninsula brought a whole new population of well-to-do San Franciscans who built homes along the Peninsula. Word of the Peninsula's mild, even climate and protection from heavy winds and fog caused more and more San Franciscans to move south and by 1900 commute trains on the Peninsula allowed residents to commute to jobs in San Francisco. A big boom in Peninsula population took place following the April 18, 1906 San Francisco Earthquake and fire. 

Population growth continued at a reasonable rate during the teens and twentys, and slowed during the depression years of the 1930s. By the end of World War II, California's population exploded as more and more people moved west. During this period more and more housing subdivisions were being built along the Peninsula, starting in the Millbrae area and moving south toward San Jose. By the mid-1950s the Peninsula was wall-to-wall housing and shopping centers from San Francisco south to San Jose. 

The growth in population along the Peninsula resulted in longer and longer commute trains along Southern Pacific's commuter line. As the population increased along the Peninsula route, the old Bayshore Highway, 101, was slowly rebuilt to a freeway and, while these highway improvements resulted in some loss in commuters on Southern Pacific, the railroad was still carrying a respectable number of people commuting to work in San Francisco. 

Early commute trains on the Peninsula were powered by a variety of power, including 4-4-0s, 2-6-0s and 4-4-2s. As trains became longer and heavier, 4-6-0s were assigned to the commute trains. By the late 1930s and early 1940s, 4-6-2 Pacific type locomotives were assigned to the heavier peak- hour trains. These Pacifics were originally purchased for main line passenger trains with the first ones, 2400-2404, built by American Locomotive Company in 1904, followed by 2405-2427 built by Baldwin in 1906 and 1907.

By the mid-1920s, SP's main line passenger trains were becoming longer and heavier and required larger locomotives. Between 1923 and 1924 Schenectady delivered twenty-eight 4-8-2s, 4300-4327, which were assigned to the main line passenger trains, which released the smaller Pacifics for secondary, branch line and commute service. 

As additional 4-8-2s arrived on SP, more 4-6-2s were assigned to commute service replacing smaller Ten Wheelers. - In 1930 SP received its first 4-8-4s, Nos. 4400-4409 built by Baldwin. This order was followed by additionaI4-8-4s from Lima in 1937 as numbers 4410-4429 arrived. Originally assigned to the Coast Daylight and other passenger trains, these 4-8-4s replaced 4-8-2s which were re-assigned to secondary passenger service and general freight service. Between 1941 and 1943 additional4-8-4s numbered 4430-4469 arrived from Lima. 

By the end of World War II the 4-8-2s were assigned to the peak-hour commute trains with 4-6-2s handling the off-peak trains, while a few 4-6-Os worked the short weekend commute trains. In the early 1950s the commute trains continued to grow with 17-car peak-hour runs requiring larger motive power. At that time the SP was well on the way to dieselizing its main line passenger trains with EMD E and Alco-GE PA units. These diesels replaced the 4-8-4s on a number of passenger runs, which resulted in several 4-8-4s being assigned to the heavy commute trains.