Here's the thing (and I hate that both sides of the argument seem to ignore this)...
Commuter rail absolutely makes sense in the Ottawa/Gatineau region, but not to the extent that some people like to imagine. The following is what our initial regional rail system should look like:
We should use existing rail infrastructure to create commuter lines with a few key stops in Ottawa's three major suburbs, and across the river to Gatineau. You start small, and then build it out as demand sees fit.
Start with a Strandherd to Bayview to Labrosse line. Hell, even give the Feds an excuse for a legacy project and ask them to create a transprovincial VIA subsidiary that functions like the GO Train if you want.
Then build the Kanata North line, and maybe a spur that leads through southern Kanata and Stittsville. You can also start thinking about a crosstown suburban line of sorts at this point (the SE Transitway and unused rail south of Walkley).
Additionally, the lure or promise of a rail line near Navan road would help spur Orleans growth south as opposed to the east, and such a commuter line would eventually make sense in the future. Beyond these lines, future expansion (since this is now at least two decades into the future) can be meaningfully explored into communities like Richmond, Smith's Falls, Arnprior, etc. This would be especially true if it's a VIA subsidiary and VIA is still providing rail service to communities like Alexandria and Smith's Falls.
Commuter rail absolutely makes sense for Ottawa right now, but only to our major suburban communities which themselves are a fair distance from the downtown core on both sides of the river. This should be the immediate focus; places like Arnprior come later after a sensible base line or two have been established to places like Gatineau, Barrhaven, and Kanata North.