The G series (Gloucester) cars were 17 m (55 ft 9+1⁄4 in) in length.
By 1960, the Toronto Transit Commission was preparing to expand its subway system to include the University line. The TTC wanted subway cars with a larger 75-foot (22.86 m) design and also wished to expand upon some of the experimental features in the existing G-series cars. The new design was pushed forward by general manager John G. Inglis.
The TTC performed testing at St. Clair and Union stations with a 75-foot test vehicle known as the Duncan Dragon. Built at the Duncan Shops by Len Bardsley and the D&D Equipment Company, the test car consisted of two trucks with three panels and was designed to test the size of train that could successfully navigate the tunnels. A steel girder with railings allowed workers to walk and ride the car during tests.
After specifications for the new cars were finalized, Alco's Montreal Locomotive Works was contracted to build the new cars, dubbed "M1". The cars are historically notable as the first subway cars produced in Canada and, at the time of their construction, the longest subway cars in the world. All subsequent TTC cars have followed the size and length specifications of the M series (though the Toronto Rocket deviates from the two-car married-pair formation) and influenced several other transit authorities to examine the use of longer cars.