For this interim service I think I would run short trains every half hour from Bramalea to Weston (arriving 3 minutes before the next UP train leaves).....I would run buses to Brampton and points west .....try it out for a while...see if the train gets any traction or not or if people prefer the existing bus schedule.
This could be run as a single shuttle train. I can think of many forms this could take, including using a two coach Nippon Sharyo DMU set, even if it is 'the spare set' so if need be, it can be pushed into service on the UPX as the back-up. Ostensibly a shuttle bus could be pressed into service as replacement to Bramalea if that becomes the case. Being a single shuttle, it would only need a single track, no switching, since there isn't any evening GO train service on that stretch, which is exactly what the shuttle is meant to supplement.
There's been more than a few times I've been forced to use the 3X bus from Guelph (Or Georgetown) to Union (transferring at Brampton) because of missing the last late afternoon train run at Mount Pleasant that gets me to Bloor Station. Missing the train means taking the bus all the way down to Union Station, and then coming all the way back on either the UPX or the TTC to Bloor (West). I'm sure I'm not alone. (If GO allowed a through bus/train transfer at Pearson Airport station, this would all be redundant, but I digress)(you can do it this way, but pay the GO fare + the full UPX airport fare)
Considering the steep subsidy offered to Airport riders on the UPX, sharing that bounty with riders to/from Bramalea with an extended 3X bus that terminates at Brampton now is easily accommodated.
The high-platforms are already extant on all tracks at Weston and Bloor. Is the necessary train transfer at Weston an inconvenience? Barely. Think Yonge and Bloor and the subway. Compare that to the alternatives. It's a no-brainer. Since this would be an alternate interspersed with the bus, it would be an hourly service, there might even be schedule time to run it between Bloor and Bramalea as a single shuttle train. Since that stretch of track is empty post PM rush hour, signalling should be a breeze, only concern being the CN local accessing the spur on the south (west) side of the tracks near Malton.
This may not show an operating profit, but it would be a huge PR and tactical plus for Metrolinx, at a very low cost, and prime the Georgetown Corridor South and connecting buses for future expansion.
What a concept...