The T1 cars entered service between 1996 and 2001 and became the mainstay of the TTC subway fleet. By 1999, they had replaced the older M1s, H1s, H2s and prototype H3s, along with some H4s, many of which had been in revenue service since the 1960s. The T1s had many of the same technical specifications of the H series, including the same married pair configuration, and incorporated many of the design elements that had been refined throughout the H-series program. Each model in the H-series production run improved on the last, adding features such as a single-handle controller for acceleration and braking, air conditioning, a change from camshaft to chopper control, and regenerative braking. The T1 built on those advances while integrating new computer technology, creating a more modern train. The T-series cars were the first TTC cars to use AC propulsion, rather than DC propulsion as used in all previous rolling stock, all of which are now retired. Other improvements included wider entry and exit doorways, flip-up seats for the installation of wheelchair positions, and the removal of vertical stanchions along the car's centre line, making them the TTC's first subway cars to be wheelchair-accessible. The interior colour scheme consists of grey floors and walls and dark red doors and panels, unlike the simulated woodgrain panels used on the predecessor H-series cars.
The T1s have a life expectancy that allows them to remain in service until at least 2026, and the TTC started implementing mid-life upgrades for the fleet including LED-type interior lighting, along with side LED destination signs and external pre-boarding route and destination announcements. As of 2019, the TTC is in the process of installing CCTV cameras on the remaining T1 subway fleet. The TTC also considered upgrading the fleet to run on an automatic train control system, which it is in the process of installing to replace the current wayside signaling system on Lines 1 and 4 and is expected to install on Line 2 by 2020 in conjunction with the opening of the Scarborough subway extension. However, due to prohibitively expensive costs of such a retrofit, it is unlikely that this will happen in the near future, and the TTC plans to replace all T1 trains with new ATC-compatible equipment instead.