In the above report, they may go with a single boring machine instead of two.
A comprehensive assessment was conducted to compare TTC’s traditional approach–using two, 6 metre diameter tunnels with each direction of track in a separate tunnel–
with the alternative of accommodating both sets of tracks in a single, large diameter tunnel.
With twin tunnels, there would be a significantly greater length of cut-and-cover construction required; the crossover track locations, the station box, and the tail track beyond the station. The assessment concluded that a 10.7 metre tunnel would be lower project cost, and would significantly reduce the extent of cut-and-cover construction as the crossovers and tail track could be constructed within the tunnel.
Given these benefits–reduction in cut-and-cover construction impacts and reduced capital costs–the 10.7 metre diameter tunnel is proposed for this Project.
With the single tunnel, the new station could be using platforms on the sides, instead of a center platform, due to difficulties diverting tracks to the sides from a single tunnel.