Twin cities are a special case of two cities or urban centres that are founded in close geographic proximity and then grow into each other over time, losing most of their mutual buffer zone. In the United States, "the Twin Cities" usually refers to the Minneapolis-Saint Paul metropolitan area in Minnesota.
There are no precise criteria for twin-cityhood, but to be considered twin cities, the cities involved have to have a similar administrative status and somewhat comparable sizes; a suburb of a much larger population center is usually not considered to form a twin city with it. For example, South San Francisco (population about 65,000) is not considered a twin city with San Francisco (population about 850,000). However, cities considered twinned by proximity do not necessarily match demographically, economically, or politically.
In many historical cases, cities that grew into each other's space lost their individual identities, and the border or barrier originally separating them became almost irrelevant. An 1873 case of twin cities merging to become a united city is Budapest in Hungary, which began as two settlements (Buda and Pest) facing each other across the Danube at a strategic fording place along a trade route. In China, the three ancient cities of Hankou, Hanyang, and Wuchang, separated by the junction of the Yangtze and Hanjiang rivers, were joined in 1927 into the single entity of Wuhan.
Minneapolis–Saint Paul is a major metropolitan area built around the Mississippi, Minnesota and St. Croix rivers. The area is commonly known as the Twin Cities for its two largest cities, Minneapolis the city with the highest population in Minnesota and Saint Paul, the state capital. It is a classic example of twin cities in the sense of geographical proximity. Together the two cities anchor the second-largest economic center in the Midwest, behind Chicago.
There are several different definitions of the region. Many refer to the Twin Cities as the seven-county region which is governed under the Metropolitan Council regional governmental agency and planning organization. The United States Office of Management and Budget officially designates 16 counties as the Minneapolis-St. Paul–Bloomington MN-WI Metropolitan Statistical Area, the 16th largest in the United States. The entire region known as the Minneapolis-St. Paul MN-WI Combined Statistical Area, has a population of 3,797,883, the 14th largest, according to 2013 Census estimates.
The 400 (later named the Twin Cities 400) was a named passenger train operated by the Chicago and North Western Railway between Chicago and Saint Paul, with a final stop in Minneapolis. The train took its name from the schedule of 400 miles between the cities in 400 minutes, and was also a nod to "The Four Hundred Club", a term coined by Ward McAllister to refer to the social elite of New York City in the late 19th century. It was an express train with limited stops between Chicago and the Twin Cities. It ran from 1935 to 1963 and spawned a number of "400" trains.
1934 had seen the introduction of lightweight streamlined trains in the United States. The railroads hoped these futuristic trains would stem the tide of customers turning away from train travel. The Chicago and North Western Railway had not invested in this new technology, but decided to upgrade track and motive power for higher speeds with heavyweight, steam-powered trains.
C&NW made their first upgrades in 1934 along the 85-mile (137 km) line between Chicago and Milwaukee, Wisconsin, introducing the 90-minute Pacemaker service to compete with the Chicago, Milwaukee, St. Paul & Pacific Railroad (Milwaukee Road) which introduced a similar train. Attention then turned to faster trains to Saint Paul: The Chicago, Burlington and Quincy Railroad ran a Zephyr demonstration train between Chicago and the Twin Cities that summer with the intent to run regular service the next year, and the Milwaukee Road introduced similar plans.