Most of New York City's subway system was privately owned until 1940. Why did the city obtain control over the lines at that point?

Pretty much the entire subway system was built by the city with city funds, but run privately by the IRT and BRT (later the BMT) until 1940. The only lines that the IRT and BMT ever built themselves with their own money were the elevated trains and surface-level trains that were eventually taken out of commission for a number of reasons (noise, and obstructed views from rich Manhattanites' apartments among them).

The city had a plan for a complete subway system before 1910, though only 70% of it has so far been built. Even today, with the building of the "T" line, the city is still using plans that were written up over a century ago. The subway lines and stations built all along the way were from an old plan created by the city, and never at the discretion of any private company.

In the early 1920s, the city was already trying to take over the rapid transit companies, since they were already doing all the underground building and expansion was proving impossible without the IRT, BMT, and the city working together. This led to the formation of the the city-owned IND line (Independent Subway System), which is now parts of the red and blue lines in Manhattan.

By that time, the IRT and the BMT were pretty much operating at a loss. Additionally, they were doing little to improve subway conditions as far as fares and equipment went, and the IRT was threatening to go bankrupt.

In order to keep the city's subway network running, the city pursued options throughout the 1930s in order to buy out the subway companies. The purchases were finalized in 1940 and that's why we have the MTA today.

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