The problem with extending the line to, e.g., Major Mac, is like a straw..
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When the straw is already too long and thin for the milkshake, making it longer only makes it worse.
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You don't need a longer straw.  You need more straws.
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This is also the problem in Scarborough.  West of Vic Park, the B-D has far too many stops, too close together.  Result ?  Even though the subway goes to Kennedy, if you are going to UTSC the fastest way is to take the YUS to York Mills, and the 95 Express across.
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Pouring $5 B into STC is no solution.
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Much better, faster, cheaper and easier to re-build the CP Don Branch to intersect the CrossTown at Don Mills, and then to run north on the Go Line to Richmond Hill.
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It avoids the slow BD; vacuums demand away from Finch, Sheppard, YMills, Lawr., and Eg, and takes it straight downtown to K, Q & Dundas.
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If you really have to  meet the BD, then build the interchange in the middle of the Bloor viaduct.
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Doesn't require any new tunnels.
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Much easier.