See
http://www.thejunction.org.uk/ under diagrams and 322 fleet Dec07. Remember there are 5 rakes and 4 diagrams (the 5th one is no longer for overhaul at Hunslett-Barclay - that is an error on the site). But one unit is in for maintenance each day (GW175), with a second (GW173) being maintained between 10 & 2 each day at Corkerhill, and GW174 returning to Glasgow at 8pm, going to Shields Depot and then forming the GW171 0705 service the following morning.
IIRC the units cycle through the 4 diagrams, such that a unit starting on G171 from Shields Depot on day 1 has 'maintenance' on day 3 at Corkerhill, before returning to Shields on day 4 and staying until day 6, when the cycle starts over.
The service is such that if any unit breaks dopwn, then the 'spare' Edinburgh unit can drag it to Shields ECS out of the normal maintenance sequence without disrupting teh service - note how GW171 sits 'spare' at Waverley from 0945 to 1714 each day! Only 3 units are actually required to run the basic service from north Berwick to Edinburgh, and if teh journey time could be cut to 30min 9perhaps if the mooted local service to Dunbar starts up, and some stops can be transfered to that service, without disrupting the ICEC / XC services, it could be possible for both servcies to ahve journey times on 30 minutes and thus teh service coudl be achieved without any increase in units, but merely through better stock deployment. IIRC teh issue is to do with capacity into the east end of Waverley which is being reserved for the future Borders rail link, whose future is far from certain.
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So that's why Glasgow gets what seems like a random, sporadic service.
Would you choose the longer journey in less comfort that can be achieved with GNER / XC via Carstairs, or FSR 170 via Falkirk if you were going end to end. The service does provide some commuter opportunity between Carstairs and Edinburgh but you would have to have very fixed shifts for it to be feasible.