Regarding the TTC, it did roll out its policy in 2010, and it was immediately challenged by the union. It actually only formally rolled it out in the field this year. The very first two tests that it did were actually positive, one for alcohol and one for drugs.
In terms of what's keeping other federally regulated companies from rolling out similar policies, well, the TTC example is a good one. Suncor is another equally good example. It tried to roll out a similar policy at a similar time and it has been similarly stuck in a judicial quagmire since then. The courts are still making up their minds about whether or not they will permit that kind of policy to go forward.
That is really the concern, that without some common national framework, these policies will keep getting rolled out and challenged on a piecemeal basis. You will get different standards applicable to different industries at different times, depending on different decisions.