While a bolton RER service that provides GO train access to western york region is fine and all, I wouldn't say it necessarily NEEDS RER. Furthermore this is a CP owned sub and the challenges to make this an RER line are basically the same as on the Milton Line, and considering how much of a headache turning that line into RER is, Bolton isn't going to get it anytime soon.
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wot?
The area around Rutherford is about as developed as Bolton, if not less. If we assume that the GO train brings a lot of suburban development, why would it be ok to develop the stretch along Rutherford and not Bolton?
Finally the point of the Bolton Line should be to connect more communities to the rail network, and improve transit throughout the network. If we want to reduce the amount of sprawl, that should be done outside of rail expansion through strong greenbelt coverage and tighter zoning laws. We shouldn't say "these people don't deserve a train because I don't want there to be more people here".
This really just comes down to how many slots CP wants to share really. If CP is willing to offer some slots, then not there is no need for this.