You mean bury them in the same tunnel? That's quite possible, considering that west of Sherway most of the subway alignments I've seen use the rail corridor for a stretch. Use the tunnel for GO trains and subway, and keep the current rail corridor for freight.
This of course would mean a much more expensive and elaborate tunnel, but if you're digging anyways, might as well go for it.
On a side note though, I think that the Eglinton LRT is much more likely to reach Square One before the B-D subway is. Building LRT tracks directly beside the Mississauga Transitway would be orders of magnitude less expensive, and it would hook in nicely with the Hurontario LRT.
Unless GO is using EMU's, that will be one big tunnel. OH!! that means the Milton to KW/Cambridge will have to be EMU also unless you are changing trains somewhere along the route. That will be about a $5 billion project since its only money.
At the same time, headways for both GO and TTC will be a nightmare from an operation point unless you have more than 2 tracks in that tunnel.
The big problem that ppl are failing to look at is where is the centre of the residential area for any line going to Hurontario St in the first place. The current RR tracks are on the south boundaries to the point it takes 10 minute walk to get to Dundas bus stops from the Dixie GO station at this time of the year. Dundas is the centre line and that is where the subway/LRT line should be in the first place.
The big problem with the dream of a tunnel below CP tracks is CP itself. CP will not be picking up any cost for relocation where station will have to be built nor allow any real interfering with its operation.
At this point in time, a BRT is only needed on Dundas for the next 20 years, but prefer to see LRT from day one up to Hurontario St. West of Hurontario only requires an BRT for the next 30-40 years since is real low density and not much development will take place along the whole of Dundas as far as Dundas for 50-100 year at the earliest.
Having an Eglinton LRT reaching Sq One before the BD is a given. The LRT would run in the BRT ROW since its supposed to be built for it in the first place. There is nothing stopping having buses use the same ROW at the same time.
Having the BD going to SQ One will have the same problem as the Yonge line going to RHC and that is the Toronto riders will never get a seat unless trains are being short turn at Colverdale Mall.
The other issue is not everyone is going to the city core, but that the way transit is setup these days to do.
You put in the Crosstown GO line, you will see a shift in ridership pattern.