The Chicago Subdivision or Chicago Sub is a railroad line in Illinois

Illinois

Illinois is the fifth-most populous state of the United States of America, and is often noted for being a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal,...

 that runs about 38 miles (61.2 km) from Chicago to Aurora

Aurora, Illinois

Aurora is the second most populous city in the U.S. state of Illinois, and the 112th largest city in the United States. A suburb of Chicago, located west of the Loop, its population in 2010 was 197,899. Originally founded within Kane County, Aurora's city limits have expanded greatly over the past...

 and hosts Metra

Metra

Metra is the commuter rail division of the Illinois Regional Transportation Authority. The system serves Chicago and its metropolitan area through 240 stations on 11 different rail lines. Throughout the 21st century, Metra has been the second busiest commuter rail system in the United States by...

's BNSF Railway Line

BNSF Railway Line

The BNSF Railway Line is a commuter rail line in the United States, provided by Metra and operated by the BNSF Railway in Chicago and its surrounding suburbs...

 commuter service. It is operated by BNSF Railway

BNSF Railway

The BNSF Railway is a wholly owned subsidiary of Berkshire Hathaway Inc., and is headquartered in Fort Worth, Texas. It is one of seven North American Class I railroads and the second largest freight railroad network in North America, second only to the Union Pacific Railroad, its primary...

 as the easternmost part of the railroad's Northern Transcon to Seattle, Washington

Seattle, Washington

Seattle is the county seat of King County, Washington. With 608,660 residents as of the 2010 Census, Seattle is the largest city in the Northwestern United States. The Seattle metropolitan area of about 3.4 million inhabitants is the 15th largest metropolitan area in the country...

. This line is known as the Racetrack because it is mostly triple-tracked and supports fairly fast trains. It had been operated by a BNSF ancestor, the Chicago, Burlington and Quincy Railroad

Chicago, Burlington and Quincy Railroad

The Chicago, Burlington and Quincy Railroad was a railroad that operated in the Midwestern United States. Commonly referred to as the Burlington or as the Q, the Burlington Route served a large area, including extensive trackage in the states of Colorado, Illinois, Iowa, Kentucky, Missouri,...

, which introduced high-speed Zephyr

Pioneer Zephyr

The Pioneer Zephyr is a diesel-powered railroad train formed of railroad cars permanently articulated together with Jacobs bogies, built by the Budd Company in 1934 for the Chicago, Burlington and Quincy Railroad , commonly known as the Burlington...

passenger trains in 1934 and ran many of them along this subdivision from Chicago to points west.

The Chicago Subdivision meets the Aurora Subdivision

Aurora Subdivision

The Aurora Subdivision or Aurora Sub is a railway line in Wisconsin and Illinois operated by BNSF Railway. It is part of BNSF's Chicago, Illinois to Seattle, Washington Northern Transcon. This segment runs about from the St...

 and Mendota Subdivision in Aurora. Commuter service ends at the Aurora Transportation Center, though 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...

 trains continue southwest on the Mendota Subdivision. Triple-tracking runs from where track leading to the Aurora station and Metra Yard joins the subdivision eastward to Cicero

Cicero, Illinois

Cicero is an incorporated town in Cook County, Illinois, United States. The population was 83,891 at the 2010 census. Cicero is named for the town of Cicero, New York, which in turn was named for Marcus Tullius Cicero, the Roman statesman and orator....

, where multiple tracks from a yard join. It is then quadruple-tracked for the rest of the way until the turn to Union Station

Union Station (Chicago)

Union Station is a major train station that opened in 1925 in Chicago, replacing an earlier 1881 station. It is now the only intercity rail terminal in Chicago, as well as being the city's primary terminal for commuter trains. The station stands on the west side of the Chicago River between Adams...

.

After the introduction of the CB&Q Zephyrs, train speeds increased significantly around the country for the next decade or so, but the Naperville train disaster

Naperville train disaster

The Naperville train disaster occurred on April 26, 1946, at the Chicago, Burlington and Quincy Railroad station in Naperville, Illinois when the railroad's Exposition Flyer rammed into the Advance Flyer, which had made an unscheduled stop to check its running gear. The Exposition Flyer had been...

along these tracks in 1946 was one event that contributed to the federal government restricting speeds in later years. Trains that had once traveled at or above 100 miles per hour (44.7 m/s) were soon restricted to a maximum of 79 miles per hour (35.3 m/s). Much of this line has a speed limit of 70 miles per hour (31.3 m/s) for passenger trains, while freight trains run slower.