To power most of its new trains, GO turned to GMDD in London Ontario. GMDD responded with an all-new purpose-built locomotive: the GP40TC. Largely based on EMDs new GP40 freight locomotive on a stretched platform, the GP40TC was powered by a 3000 turbocharged 16-645 prime mover with a smaller EMD 12V-149 for HEP.

Constructed in 1966, these were delivered several months in advance of the start of service, and most were pressed into freight service by CN. Painted solid dark blue with red railings and a small white CN noodle on the cab, these were regularly seen in freight service in Southern Ontario. Early in 1967, they were returned to London for the installation of the HEP equipment. Repainted in full GO livery in the spring of 67, all 8 units were on hand for the beginning of service on May 23, 1967.

Over time, GO renumbered and made several modifications to all the GP40TC fleet. This included modifications to the HEP setup with muf