The Westcoast Express has been operating since 1995 between Mission and Downtown Vancouver and by everything I have seen, it seems to be a reasonably cost effective commuter transit solution. It makes me wonder of if there are other places in Metro Vancouver where you could operate a similar service? I think there are several more that could be used.
In order of how viable I think they could be:
Southern Railway of BC from Abbotsford to Scott Road in Surrey
This rail line is about 55 kilometers long and really runs through the heart of major populations centres in the Fraser Valley. There should be more than enough potential customers to make it viable. The one issue is the destination, the line would effectively have to end at the Scott Road SkyTrain station, as would other suggested lines.
Many of the existing rail lines in the lower mainland coverage at one location, the rail bridge over the river at Scott Road. This rail bridge over the Fraser between Surrey and New Westminster is a single track which makes for a major constraint. Even if it were double tracked it would still not be very useful because on the north side of the river the rail route is not very direct or useful. Scott Road SkyTrain station is the obvious end point.
Abbotsford to Scott Road should take about 50 to 55 minutes assuming about six stops along the way. It would then take another 33 minutes on the Expo Line to get Downtown. This would make the trip from Abbotsford to Downtown via this route about 10 minutes longer than the current Mission to Downtown on the WestCoast Express.
The line would be beneficial for people living in Langley and Cloverdale. A train from Langley to Scott road should take about half an hour.
What this line does not do is serve the commercial core of Surrey which is now a significant commuter destination now. Still, I think this is the only line that has any real potential to be viable and there is a group pushing for this to happen.
BC Rail Line from North Vancouver to Squamish
More and more people are living in Squamish and commuting into Vancouver. The upgraded highway is nice, but the BC rail line is right there and could offer direct service to Lonsdale Quay.
It is 63 kilometers from Squamish to Lonsdale SeaBus terminal in North Vancouver. With only a couple of stops along the way, say Lions Bay and Horseshoe Bay, it should be possible to do this route in about an hour.
Squamish and the whole Sea to Sky corridor have been booming in population for years now. Since there does not seem to be any expectation of this slowing down anytime soon the potential customer base is likely to rise. The problem is that the population is likely not large enough yet to fill enough seats.
BNSF line from White Rock to Scott Road
This 30 kilometer rail line skirts the edges of neighbourhoods in White Rock, South Surrey and North Delta. It may not be close enough to the public to offer the demand it would need.
CN line south of the Fraser
The line runs from Chilliwack along the south bank of the Fraser all the way into Surrey at Scott Road. The Line is actually a much more direct line to Scott Road but for first 50 km from Chilliwack the line does not pass by any large population centres. Even the last 20 km from Fort Langley to Scott Road the line is not really in a useful location for commuters. In fact for a lot of the route through Surrey it is inaccessibly located along the Fraser at the bottom of a steep hill.
This line would only make sense if there was a large demand in Chilliwack for a commuter service. Chilliwack is not nearly large enough in population and does not have a great many people commuting into Vancouver.
Other Rail Lines
The rest of the rail lines in the lower mainland do not look like they could offer any commuter benefit because they do not run anywhere close to where commuters live or where they want to go to.
Kelowna to Vernon - yes, not in Metro Vancouver
There is one more place where a rail based transit system might work, in the Okanagan. The line runs from Kamloops to Kelowna. In Kelowna the line is perfectly situated to offer a quick service from downtown out to UBC Okanaga, the airport and Winfield. It is a nice straight running closely paralleling highway #97. It could offer a commuter service all the way to Vernon. The problem is that the line is in the process of being abandoned.