each door opening. Cars 2250–2299 were different in that each magnet controlled a separate door leaf. Experimenting with new technology A handful of A/B Standards became the first New York City subway cars to experiment with a public address system. Car 2369 was the first to be so equipped; ultimately, several cars received a loudspeaker telephone system in 1923–24 to aid conductors in making announcements. The experimental setup was removed by 1928, but the idea was ahead of its time. PA systems did not come into widespread use in New York's subway until the 1950s. The A/B Standards were also the first New York City Subway equipment to experiment with cab signaling. The underlying rationale for the experiment was to allow trains to run safely at closer headways to provide more frequent service. An attempt was made in 1916 on car 2148 to test a GRS cab signal system using A/B Standards. While generally working as intended, the experiment did not have staying power. The equipment necessary for its use was removed by 1918. It was not until the 1990s that a similar idea would be revisited in the subway, when MTA New York City Transit installed modern CBTC signal equipment on the BMT Canarsie Line. Letter designations Over their service life, the A/B Standards used several letter designations depending on the configuration of the cars. Originally, the first 600 cars as delivered could operate singly and dubbed A-types. During the 1919–1920 modifications, much of the fleet was reorganized into semi-permanently coupled units as an MUDC system was being added. The following configurations refer to the cars in operation over the years: A units were motorized single cars and capable of running independently. B units consisted of three motorized cars that ran in a set. The two end cars retained their operating cabs at the front and rear, but blind cabs were made inactive. Door control button boards were similarly deactivated in the end cars, but retained in the center (master) car. The center car's blind cabs were made inactive. BT units consisted of two motorized cars that ran in a set. Each car retained its operating cab at the front and rear, but blind cabs were made inactive. On these units, there was no provision made for door operation, and thus, these units could not be operated independently by themselves. (Note: this designation was also used much later on