Rail to Renfrew seems to me to have the problem that it's going to take up further valuable space on the Glasgow-Paisley corridor and in GLC station. This was one of the problems that I had with GARL and its frankly silly plans to take four non-stop paths an hour between Glasgow and Paisley and run to Glasgow Airport, almost certainly with very few passengers, whilst the busy lines in Ayrshire were denied room to grow. (The idea of a rail line to the airport was, of course, good. The reality of that particular plan was not.)
The infrastructure already in place sees a line diverge just West of Cardonald. A new line diverging West of the Hillington stations looks unlikely to find a way into the centre of Renfrew, so is unlikely to take over the local calls in Hillington (if I could bulldoze a few dozen homes I'd run a local service to Renfrew via Hillington, but I don't think the locals would appreciate it too much!).
This is where Crossrail becomes quite useful, I'd argue. Running to Renfrew without clogging up Central would be an attractive solution, although I do wonder if not calling at a central Glasgow station would weaken the business case for a Renfrew station? Nevertheless, it will have an interchange with the North Clydes at Bellgrove and could have one with the Argyle line at Glasgow Cross, and the Subway at West Street which would help a lot. As would a direct rail link to Braehead (on a side note, I think the public transport links to Glasgow's three "new" out of town shopping centres at Braehead, Silverburn and Glasgow Fort are a disgrace). Could this then be continued to Glasgow Airport, providing an alternative GARL solution?
Relocating High Street and using the proposed curve is yet another expense, but a Charing Cross - Renfrew service would again be a possibility if a turn back was provided at Kelvinhaugh.
TBH though, Crossrail is a bit of a messy solution to Glasgow's core rail problem, which is a disconnect between the North and South of the Clyde. I don't think it addresses the problem areas as well as some would like us to believe. It will see trains bypassing the city centre and attempts to get trains to Queen Street are going to simply lengthen journey times at quite considerable expense (I suspect relocating High Street Station in to a tunnel is not going to come cheap). It will do little to alleviate the problem of poor links between Central and Queen Street. A lack of paths through Partick, and a lack of space to expand Partick station mean that trains are going to need turned back at Exhibition Centre or Kelvinhaugh, which dilutes benefits further by removing direct rail access to the popular West End of the city.
What we've seen with Cumbernauld being re-routed to GLQ LL is that it's extended journey times, and I'm pretty sure that it's going back to the High Level when electrification is done. This is because they didn't build the Garngad chord (which would have bypassed Springburn) and I suspect closing Springburn station (in one of the most deprived areas of Glasgow) would be politically sensitive, even with a relocated station on the chord. Furthermore, Robroyston station is in the pipeline, and if the Cumbernauld runs were permanently re-routed it would have very few passing trains which could stop there!
I suspect that any plans to reroute Renfrewshire commuters into the low level at Queen Street would be met with similar resistance. It would be slower than going to Central HL and therefore shunned in favour of the direct fast trains which would continue to pick up the slack (although Renfrew wouldn't likely have that problem, being a new route). So, 2tph Renfrew-Springburn and 2tph Renfrew-Charing Cross(-somewhere in the West, if paths allow) might work, but would be expensive.
Another question - does Renfrew really then need a heavy rail link? Getting trains there is clearly no easy feat, and I can't find an elegant solution that doesn't create capacity problems. But Renfrew should at some point in the future be getting Clyde Fastlink, which is really a diluted plan for a Glasgow Tram line. This was heralded as a key part of the Commonwealth Games transport plan, and quietly forgotten about. (SPT's website still proudly boasts that it will be in place for the Commonwealth Games... well done). It may not be rail, but an off-road bus line from Glasgow City Centre - Partick/West End - South Glasgow University Hospital - Braehead - Renfrew - Glasgow Airport could be a competitive solution. And it would solve plenty of problems. It would be a fast and frequent route from Renfrew to Glasgow. It would provide connectivity between the West End and the South Side. It would provide an excellent link to the new South Glasgow University Hospital (the existing Southern General has dire bus links, particularly to the West End). And it would provide excellent airport links. I think a light rail network would be better, but Scotland is unlikely to ever support one again in my lifetime after that fiasco in Edinburgh.
Just my two cents.
As for the other proposals in last year's article... Lossiemouth? Really? It's like a ghost town. It's quite nice in its own way but I can't see it ever warranting a rail link. However, with a proposed 2tph Inverness to Elgin (with one continuing to Aberdeen), it could perhaps be justified if and only if the proposed Elgin terminator has nowhere else to go for long enough to allow it to jaunt down to Lossie, and if the line can be built cheaply (single track the whole way should suffice).
St Andrew's (presumably to Edinburgh) has more legs, but has a similar issue to Renfrew in that you'll be taking away paths through Fife to serve the town. If you could find a way of providing the service without depriving the existing service (most notably, 2tph Edinburgh-Dundee), then perhaps it would work.
Haddington isn't likely either - well off the ECML so you'd either be diverting trains (and slowing them down in the process) or adding new trains to an already busy line. Staying with Edinburgh suburbs, Penicuik wouldn't be a bad suggestion, but this would be a completely new route which would need a strong business case. But perhaps an extension of the Newcraighall line would be a possibility, calling at Eskbank (new station with Borders Rail Link) and then turn out via Bonnyrigg. Would Newcraighall and Eskbank warrant 4tph though?
Glasgow - Paisley - Linwood - Bridge of Weir - Kilmacolm would suffer from similar issues to Renfrew, but again if you can overcome this I suspect that the line could be justified (perhaps divert Paisley Canal to GLQ LL as well as all the above?).
Most of the 50 stations proposed, though, are a bit easier to achieve, and would be new calls on existing services. Here's a
link to a map.