Im only speculating but one would imagine that both STC and the Kennedy -> Unionville portion of Smarttrack would NOT need 5 minute frequencies to be successful.
I believe the 5 minute frequencies were required to bring relief to the Yonge subway.
I dont see either the STC spur nor the Kennedy -> Unionville section being part of the relief of Younge. Relief of Younge/Bloor would come from the Dundas West Station -> Main Street Station section of Smarttrack.
So one could surmise that offering 5 minute frequencies to Lawrence Smarttrack station, and then splitting off to STC and Unionville with 10 minute frequencies would be sufficient.
A while ago I took a look at the ridership numbers of Bloor-Danforth. If memory serves me correctly, nearly half of the ridership east of Yonge got on at Kennedy, Warden, and Victoria Park (if it wasn't exactly 50% it was surely a pretty sizeable percentage). By basically taking all of the current SRT riders out of Kennedy, right away you've relieved Bloor-Yonge quite a bit.
What train capacity are you using - maybe 1000 passengers.
10 minute frequency is 6000 ppdph.
The combined
Eglinton-Scarborough Crosstown had about 12000 ppdph demand and the was SRT just to McCowan. They need to provide 5 minute frequency to STC - possibly 4 minute.
Let's do the math here. The projection for the Scarborough Subway pphpd that I could find was 9,000 in 2031. Let's use 10,000 to be safe. We can assume that a SmartTrack spur to STC would draw about the same.
Let's assume RER would use the Stadler KISS (
https://en.wikipedia.org/wiki/Stadler_KISS). If not, it will likely end up being something similar, with a similar capacity. The seated capacity of a 6-car train is 435, while the standing is 838 (
http://www.railway-technology.com/projects/stadler-kiss-double-decker-trains/). Let's round down to 700, since we don't want it completely at crush load at the start of the line.
To accommodate 10,000 pphpd, you'd need a train every 4.2 mins. If we were to go with the actual projected ridership of 9,000 pphpd and the crush load of 838, it'd be a train every 5.6 mins. So yes, you'd need a train every 4.5 to 5 mins in order to satisfy the demand.
So basically, if you were to cut the Kennedy to Unionville section out of SmartTrack and have it divert to STC instead, you could accommodate the projected 2031 ridership. If you were to run the Kennedy to STC shuttle route every 10 minutes on top of that, you'd have no problem handling the demand.