While I don't doubt that ridership on the rail line would be a bit too low to justify, I HATE, HATE, HATE this ideology that "well, lets examine current ridership and assume that future ridership will be the same based on that"
NO! Thats not how future prediction works. You have to take into account all aspects, including how the new service will increase demand for ridership.
Perhaps people don't like taking the bus. Perhaps many people who drive to Toronto from Orangeville would be incentivized to leave their car at home to take a train.
Most people who take the bus take it because they have no other choice. People who drive take the train. Theres a simple reason for that, beyond the perception of buses being less desirable than trains; trains don't get stuck in traffic.
Theres so many variables to look into when determining if a project is worthwhile, and the predicted ridership, you cannot, cannot just look at the existing system and say "well no one rides this so no one will ride X". Perhaps the current system sucks...