With the Barrie Line undergoing double-tracking, it looks like it could beat the Kitchener line to being the second line with all-day service. But in the meantime, we could actually introduce a few additional round trips using the existing infrastructure to capture some demand outside of the peak period.
I came up with this schedule, which shows how we could add three new round trips to the existing peak-direction peak-period trips within the constraints of the current track layout (single track line with a passing siding north of Maple):
While 3 additional round trips doesn't seem like much, I think they would actually go a long way toward making people consider GO transit as an option for general travel, not just commuting. Barrie and Toronto are 100 kilometres apart so there is likely a fair bit of inter-city travel demand, which is not restricted to peak periods. 8 daily trips per direction would already make the Toronto-Barrie corridor one of the highest traffic intercity rail corridors in the country. Even if we account for 5 of the trips being around the same time, it is still effectively 4 or 5 trips per day, which is fairly typical for inter-city train service in the Quebec City - Windsor corridor.
The midday trips skip stops which I wouldn't expect to have demand for off-peak travel. These are mainly isolated park-and-ride stations, which would already be full by that point anyway. This makes faster off-peak service which would be a competitive inter-city option.
Downtown commuters would also benefit from the new services. The midday trips would allow greater flexibility in hours, and would likely be particularly popular on Fridays when some people work half days. The new evening northbound trip could serve downtown workers who choose to stay downtown later, perhaps going out for supper.
Technical details about the schedule:
I made it using the travel times from the existing schedule, and assuming 2.5 minutes saved per stop skipped by express trains. This value is based on other GO express train schedules.
Maple delay accounts for the fact that trains using the siding are delayed by a few minutes according to the Barrie Line summer weekend schedule. Maple Gap is the time between when a northbound train leaves the (single-track) station and a southbound train arrives.
Padding refers to the time between when a train arrives at a terminus and a different train departs. I included it so that the arriving train will not delay the departing train if it is a bit late.
For the most part, the off-peak trips consist of a single train running back and forth down the line. The exception is trip F2, which pulls over into the Maple siding to let trip B2 past. The schedule does use 6 trainsets instead of 5, but trainset B is unoccupied between 7:40AM and 8:00PM, during which time it could run on other lines.
