Building high-speed rail to serve the 
Quebec-Windsor corridor is one of the most important investments that I can imagine for Canada. 
This article in Fast Company looks at some projected benefits for HSR in the US based upon the experience in the EU. 
In the crowded Northeast Corridor (the perfect place for high-speed rail), 62% of the people choosing between taking a train or a plane from Washington, D.C. to New York pick the train, as do 47% of Boston to New York travelers. And the people who choose the train instead of driving would decrease wear on the roads, resulting in $270 billion in road repair savings by 2050. That pays for the trains right there. And lest we forget the environmental benefits, remember, trains are the most efficient mode of passenger movement, especially high-speed...
Now, the Canadian situation is even more interesting because the Quebec-Windsor corridor is home to probably 60% or more of the country's population. There is literally no other infrastructure project I can think of with the business and environmental benefit that would provide. Can you imagine making the trip from Montreal to Toronto in a little over two hours [
estimate source]? 
Despite its low cost (est. $2.6 billion) I don't consider the 
ViaFast proposal to be all that compelling for the Quebec-Windsor corridor because trains still have to compete with freight rail using the same tracks. An estimate for a dedicated, electrified track (presumably serving a 300 KPH rail service) is given in 
Queen's Policy Review  (QPR) at $25 billion [PDF].
In my 
last post I noted that tax cuts are a poor investment for creating a sustainably attractive business environment. The estimates given by the Conservatives put the cost at $10 billion between 2011-2012 and 2013-14 (the Parliamentary Budget Officer pegs it higher, at $11.5 billion) [
Globe & Mail]. In the Fast Company article, they quote a 
recent report [
PDF] by the American Public Transportation Association which notes that "for each $1 billion invested in high-speed rail projects, the analysis predicts the support and creation of 24,000 jobs" and all the taxes they pay. The QPR report, mentioned above, also notes the annual $170 million subsidy to VIA to make up their shortfall which should be considered, and the more-than $1 billion in subsidies to Bombardier since 1982 to keep it afloat despite it being one of the finest train manufacturers in the world. Finally, there is the cost of traffic congestion, 
estimated in 2009 by the OECD for Toronto alone at $3.3 billion per year which, granted, will not be wholly solved by HSR, but it will help. That's got to be half the money needed right there. If Ontario can find $4 billion to prop up GM and keep manufacturing jobs from evaporating entirely, surely we can find some money to invest to rejuvenate manufacturing in Ontario and Quebec while accomplishing one of the most important public infrastructure projects in the history of this country.
More Canadian HSR resources:
http://highspeedrail.ca/
http://highspeedrailcanada.blogspot.com/