Cal-P is an unofficial term for the railroad line from Martinez

Martinez, California

Martinez is a city and the county seat of Contra Costa County, California, United States. The population was 35,824 at the 2010 census. The downtown is notable for its large number of preserved old buildings...

 to Sacramento, California

Sacramento, California

Sacramento is the capital city of the U.S. state of California and the county seat of Sacramento County. It is located at the confluence of the Sacramento River and the American River in the northern portion of California's expansive Central Valley. With a population of 466,488 at the 2010 census,...

 (or perhaps for the whole line from Oakland to Sacramento via Martinez). (Officially the line is the Martinez Subdivision from 10th St Oakland to Roseville.)

It hosts several Amtrak

Amtrak

The National Railroad Passenger Corporation, doing business as Amtrak , is a government-owned corporation that was organized on May 1, 1971, to provide intercity passenger train service in the United States. "Amtrak" is a portmanteau of the words "America" and "track". It is headquartered at Union...

 passenger routes, such as the Capitol Corridor

Capitol Corridor

The Capitol Corridor is a 168-mile passenger train route operated by Amtrak in California. Because it is fully supported by the state, the Capitol Corridor operates under Amtrak California. It runs from the San Francisco Bay Area to Sacramento, roughly parallel to Interstate 80...

, Coast Starlight

Coast Starlight

The Coast Starlight is a passenger train operated by Amtrak on the West Coast of the United States. It runs from King Street Station in Seattle, Washington, to Union Station in Los Angeles, California. The train's name was formed as a merging of two of Southern Pacific's train names, the Coast...

, and California Zephyr

California Zephyr

The California Zephyr is a long passenger train route operated by Amtrak in the midwestern and western United States.It runs from Chicago, Illinois, in the east to Emeryville, California, in the west, passing through the states of Illinois, Iowa, Nebraska, Colorado, Utah, Nevada, and California...

. The name comes from the California Pacific Railroad

California Pacific Railroad

The California Pacific Railroad Company was incorporated in 1865 at San Francisco, California as the California Pacific Rail Road Company. It was renamed the California Pacific Railroad Extension Company in the spring of 1869, then renamed the California Pacific Railroad later that same year...

 (Cal-P), which built the line from Suisun-Fairfield to Sacramento (so some would say "Cal-P" only refers to that piece of railroad). The Cal-P was taken over by the Central Pacific, which in late 1879 completed the line from Suisun-Fairfield to Oakland via the train ferry from Benicia 38.045°N 122.166°W to Port Costa 38.048°N 122.183°W. The present double-track lift bridge across the Carquinez Strait replaced the train ferry in 1930.

The California Pacific became part of the Southern Pacific Railroad

Southern Pacific Railroad

The Southern Pacific Transportation Company , earlier Southern Pacific Railroad and Southern Pacific Company, and usually simply called the Southern Pacific or Espee, was an American railroad....

 (SP) which in the 1990s merged into the Union Pacific Railroad

Union Pacific Railroad

The Union Pacific Railroad , headquartered in Omaha, Nebraska, is the largest railroad network in the United States. James R. Young is president, CEO and Chairman....

(UP), the line's current owner. UP operates numerous freight trains over the route.