to designate cars formally from BX units that had their trailers removed but did not receive a former A car as a replacement.) BX units were three-car units consisting of an unpowered trailer between two motorized cars. The two end cars retained their operating cabs at the front and rear, but blind cabs were made inactive. There was a restriction on the use of these units; one such unit was not permitted to operate by itself, nor could two of these units operate together in one consist. In addition, for many years they were forbidden to operate on the Manhattan Bridge, but this restriction was lifted when the majority of the units were reassigned to Coney Island to provide for the expanded service via the 60th St. Tunnel Connection on December 1, 1955. AX units were trailers that operated in connection with an A-type car, loosely associated. Five trailers were originally set up in this manner, but the arrangement was not found to be satisfactory, and they were ultimately coupled into BX units as the rest of the trailers already had been. Though they did not themselves have motors, the cabs could control the entire train's propulsion. They were typically added onto trains to lengthen them during the rush hours, and they were used strictly on the 14th St.–Canarsie Line. The frequent operating and shop personnel references to the cars as ABs or AB Types were derived from the above letter designations, as the vast majority of the cars were originally A units, and later reorganized into B units. Additionally, it is noted that other letter designations were used temporarily involving an extra "A" preceding the unit name. This was done as the A/B Standards were being modified in regard to their jumper cable connections. This work went on from approximately 1928 to 1931, and during those years, one could see units carrying designations "AA", "AB", or "ABX". The extra "A" signified that the car had been so equipped. These designations with the extra "A" prefix were temporary, and when the entire fleet of these cars had been so equipped by 1931 or 1932, the extra "A" prefix was dropped from the designations. Regarding the first 600 cars, while they were being unitized, it was found that 2 cars of this series (2399 and 2500) did not fit into the system as it was being set up. These were relegated