With the exception of the extension of platforms towards George Square (a suggestion that probably won't happen), I don't think Queen Street's platforms can be extended much, if at all. The only thing that could be done is an extention of P7 towards the tunnel, but that's unlikely to ever be used by Scotrail, and HSTs would still block the tunnel.
As for HSTs in Queen Street, they will overhang even on Platform 7. A look
here will also show you that a long HST will block the valuable Platforms 5 & 6. Queen Street needs all the platforms it can get, particularly Platforms 2, 6 and 7 which can hold 6 car trains. So you'd be crippling the station by putting an HST there!
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My understanding (for what it's worth) is that Bellshill was chosen partly for its proximity to Motherwell
Back in 2007, Uddingston station was the plan. (See Page 10-11) The stopping pattern planned was the same as today, albeit with Uddingston replacing Bellshill. Uddingston would actually be the best option IMO. It would allow people from Hamilton and Blantyre to easily drive to the station (or take a bus) and catch the train. On the other hand, Bellshill isn't the best (but in my experience is quite well used).
In fact, in the medium term, a "Holytown Parkway" station had been planned to serve Ravenscraig and drivers from the Lanarkshire area and tempt them onto the trains
instead of stopping at Uddingston. The aspirational service frequency at this stage is also 2tph semi fast plus the local. It might seem a bit much, but if you have a lot of use from Lanarkshire plus connections from Glasgow Central (eg, Paisley, EK, Kilmarnock...) it would actually work quite well.
More recently, in
January 2009, the stop had been changed from Uddingston to Bellshill because Uddingston is on the WCML and, apparently, stopping there would slow down the mainline. So, instead, the Lanarkshire stop was moved to Bellshill despite the benefits above.
To be honest, though, I can't see why that's a problem. There's a departure at xx:00 for Birmingham every other hour (even hours only), and a (generally) hourly departure at xx:40 for Euston. So I can't really see why stopping at Uddingston would cause such a problem in either direction.