...the old dual frequency era when there was a good chunk of Ontario still on 25 Hz power...
That's interesting and I've never heard of it before. The German and Austrian mainline railways still run with 15 kV at 16 2/3 Hz, the reason for that being that the early traction motors suffered from excessive brush sparking at 50 Hz.
Same in Schweiz, Norway and Sweden too. But not Denmark; they did not have any catenary mainline until the late 1980s, opting for steam and diesel, so when electrifying they went with 50Hz 25KV instead. For the 16
2/
3Hz there was a mix of supply systems in play; some hydroelectric generation up north in Sweden was single-phase and correct frequency, but the most lines were fed by rotary conversion plants, frequently installed on railway cars parked in bedrock tunnels along the lines. In the 80s, semiconductor conversion became possible, but rotaries were used pretty long after that.