The discrepancy appears to come between conflicting stated goals between the Big Move (full two-way, all-day service to Milton within 15 years) and GO 2020 (two-way, all-day service only to Meadowvale station in 15 years, and to Milton and Lisgar within 25 years). An objective of The Big Move Update is to align the GO 2020 plan and The Big Move. Reports cited "significant infrastructure and operational challenges that mean it will not be possible to deliver two-way, all-day service all the way to Milton in the 15-year time horizon. Additional tracks and potentially numerous grade separations are necessary are a prerequisite to the expansion of service to Milton. The construction is especially challenging through built-up areas. This rail corridor is largely owned by CPR, a private third party operating freight rail. Their approval is required for any service and infrastructure expansion. Two-way, all-day service can be delivered to Meadowvale in the 15-year timeframe, but the full extension to Milton can only be delivered over the 25-year horizon."
