LRVs in Tram-Trains would be the choice if that route were taken, and then they could be inter-operable with the Finch and Eglinton lines, albeit the choice of catenary power might dictate dual-voltage (25kV AC/750v DC) vehicles, which is done in other jurisdictions. TC might have stipulations on separating the track with the heavy rail operations. That's a big question pending with VIAs electrification, so a big change on Transport's part might be in the offing. It certainly shouldn't be "given" to the TTC, however, as with the crosstown LRTs, they might have some operating arrangement, but then that complicates overlapping staffing for stations that are already GO staffed.
Edit to Add: Here's one in the UK with an actual British Rail Class number:
[...][The 37m-long dual voltage (25kV ac 50 Hz/750V dc) vehicle, which is designated class 399, will be shipped from the port of Santander to Southampton, where it is due to dock on November 28, before reaching Sheffield on December 1.][...]
http://www.railjournal.com/index.php/light-rail/vossloh-despatches-first-sheffield-tram-train.html
[...][The introduction of tram-trains will allow Supertram services to travel on the national rail network. Passengers will be able to board one of the new vehicles at Sheffield Cathedral and travel directly to Parkgate Retail Centre in Rotherham, via Meadowhall South and Rotherham Central station, in about 25 minutes.
It is the first tram-train to be built for the UK, but the project is drawing on the experience of tram-train systems in Europe, specifically Karlsruhe. The Karlsruhe Model is often cited as the first tram-train system in Europe and has been the basis of other projects around the continent. The German city has also recently begun operating a new fleet of Vossloh Citylink tram-trains similar to those being trialled in Sheffield.
[...]
What really makes a tram-train is its ability to adapt to different electrification and signalling systems. Sheffield’s Class 399 vehicles are dual- mode, allowing them to operate under the 750V DC catenary on the Supertram network and the national network’s standard 25kV AC OLE. The route to Rotherham will be electrified at 750V DC – a project due to be completed by the end of 2016 – but a dual-mode vehicle was seen as essential given plans to electrify the Midland Mainline to Sheffield by 2023.
The switching process is automated. An Automatic Power Control (APC) system, which uses magnets embedded in the ground outside of the rail, separates the two power supplies with a neutral section of track. As the vehicle travels over the first magnet it triggers the circuit breakers to open. The tram-train then coasts through the neutral section before detecting the new voltage and closing the circuit breakers. Although it is an automated process, there is a manual override which would allow the driver to close the circuit breaker if needed.
[...]
http://www.railengineer.uk/2015/12/23/sheffield-tram-train-making-square-pegs-fit/
The US (Caltrans and other jurisdictions) is already allowing mixed light and heavy rail, in Caltrans case at the same time, change is going to have to come to Canada too.