I posted this on another forum and can't be bothered to write it all out again, but the comments are equally relevant to the debate here.
I've spent some considerable time running through the potential impacts of the various stages of E&G and other electrification schemes (Whifflet/Cumbernauld, plus expected Shotts route, East Kilbride and Barrhead) on ScotRail's DMU fleet. Without going into too much detail, the combined effect of those schemes is that all 48 156s can easily be replaced progressively up to 2019, if the wires go up as expected. Units in the Strathclyde area are replaced by EMUs, naturally, whilst the West Highland could be seen off by 158s - my suggestion being that the 15 current Haymarket ones (158726-736/738-741) are refurbished to a similar standard as the Inverness batch (701-725) to make them more suitable for tourist traffic. The GSW Stranraer, Kilmarnock, Dumfries & Newcastle services (for which something like 12 units are required) could be covered by displaced 170s or the 8 Haymarket 'extra' 158s (782/86/89/867-871) being retained, thus a pool of 23 158s to cover both GSW & WHL. Naturally, route clearance and driver training required, but that will be inevitable if the 156s are to go. On top of the 48 displaced 156s, the electrification projects noted should allow for something like another 8 158s or 170s to be displaced, even allowing for a bit of extra capacity and the Borders line.
What I haven't worked though is the potential impact of the deploying HSTs on E/G-A/I services, which will invitably have a large impact on 170 requirements. We do not know if all services will go over to HSTs, personally I doubt it, but it is reasonable to expect something in excess of 20 170s to be directly replaced on these 'ScotRail Express' services. Common opinion seems to be that the 170s are not particularly suitable for the WHL and Far North/Kyle given their gearing and lack of horses for the hilly bits, leaving displacement of a few extras to the Inverness-Aberdeen line as a possibility (my interpretation being that HSTs won't see use on this line, but I may be wrong on that) along with Edinburgh-Perth/Fife/Dundee/Borders becoming solid 170 territory. I doubt retaining the existing fleet of 59 170s can be justified.
Summed up. Something like 70 to 80 DMUs to be displaced, starting progressively from when the E&G main line goes electric in 2016/17 and trickling through as each other project goes 'live' and HSTs arrive, running to 2019/20 and all subject the usual delays and complications. Those 70 to 80 DMUs will include the 48 156s with the balance made up from 158s and 170s in a variety of manners, dependent on what redployment options are chosen for the displaced units.
As for the EMUs, 80 seems a high number, but something around 40 for E&G/Dunblane/Alloa seems about right, with the balance sounding like they are to replace the 30 or so 334s used on the Helensburgh-Edinburgh via Bathgate services. The 334s in turn displace the 314s for scrap, with 318s taking over their turns and add to the extra capacity required to cover requirements for East KIlbride, Barrhead and the Shotts route.
Naturally, any surprises like new stock for the West Highland, changes the situation somewhat.