It depends
While I am supportive of rapid transit in the 401 corridor, I don't think LRT or subway would likely be the right application.
I also think a 'modified' alignment would be needed to make to work.
To explain.
Roughly, and this is not strict as on any line, you can tinker with rolling stock and station spacing.....the assumption usually is:
Bus-hyper local, frequent stop service
LRT/BRT slightly large stop spacing, some district/regional demand
Subway - Heavy demand, all-day stop spacing 2-4x that of LRT/BRT
Commuter Rail, long-distance, express service, 4-6x stop spacing vs. lrt
Freeway corridors do not typically (there are exceptions) generate LRT/local type demand, nor would it be practical to accommodate it. Stations generally would need to be near bridges, that means much greater stop spacing. Besides, highways don't tend to serve local demand.
Subway could work, but again, there are spacing issues, density issues, trip generation issues around that type of service, and barring a 4 track alignment, it would not meet express demand well.
So the Commuter Rail (GO Train option) seems best (though it might well use EMU rolling stock)
But, you still have to address direct connections to trip generators.
I don't think a service will do well, if you have a long outdoor platform, a climb to a highway overpass, which isn't very pedestrian friendly, probably a few blocks from your destination, where you would likely wait outside for a bus.
So, I would suggest the following, and underground station at Pickering GO Station, tracks emerge into the mid-highway corridor (to save money); but the trains go underground, briefly to divert south at Scarborough Center, and meet the RT Station; then divert back into the highway (at grade) before going underground again at Yonge, and having a direct connection to Yonge-Sheppard Station. Back to the Highway, then a diversion to meet Wilson Station underground at the Spadina Line, before heading express to Pearson Airport.
An additional Connection to Oriole GO Station could be made, with a mid-highway station.
A connection to the Stouffville GO line would have to be demand-tested, as it would only make sense to divert north and meet Agincourt Station underground.
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Having said that, I don't this ranks as a top priority.
Among Commuter projects, I would rank:
1) Lakeshore to REX
2) Georgetown to All-day/REX
3) Barrie to All-Day
4) Stouffville to All-Day
5) Milton to REX
6) New GO Mid-town connection (E-W) this could be the CP midtown line or a #401 line
So I think there's lots to do before we get to this project, but I do think it has a place in the future.