In 1940, the city bought the two private systems. Some elevated lines ceased service immediately while others closed soon after. Integration was slow, but several connections were built between the IND and BMT; these now operate as one division called the B Division. Since the IRT tunnels, sharper curves, and stations are too small and therefore can not accommodate B Division cars, the IRT remains its own division, the A Division. Many passenger transfers between stations of all three former companies have been created, allowing the entire network to be treated as a single unit.