Based on the 75ft M1 the early H-series cars improved on the design, notably by enlarging the operator's cab and using a single-handle controller. Over the programme revisions were made to the designs and each production model in the H-series improved on the last. The H5's were the first to use "chopper controls" . The H5's also added regenerative braking and air-conditioning which was also incorporated into all future designs. The completion of the H6's allowed the retirement of the G class cars and no further H-Series orders were made. A prototype new car T-series was built by UTDC in 1990-91, and evaluated by the TTC. By the time the TTC was ready to order new cars in 1992, UTDC had been sold to Bombardier. Bombardier then constructed the new car order – which would add new technology such as AC propulsion to a similar platform based on the predecessor H-cars.