BEMUs*
The BEMU plan is particularly interesting. The 222s are coming to Scotland, and everybody knows that they are. Once the 222s arrive, there will be enough, and reliability should be good enough, to run pretty much everything between the Central Belt and Inverness/Aberdeen with the 222s, prioritising the Inverness services first.
That buys you enough space in the 158/170s to start splitting diagrams across the country at Perth and at Arbroath. That leaves you with a sizeable chunk of the fleet confined to services across the Central Belt. Currently the 158/170 fleet runs the following services:
2tph Edinburgh - Tweedbank
1tph Edinburgh - Cowdenbeath
1tph Edinburgh - Leven via Dunfermline
1tph Edinburgh - Leven via Kirkcaldy
1tph Edinburgh - Perth via Ladybank
1tph Edinburgh - Dundee via Kirkcaldy
1tph Glasgow Queen Street - Dundee (some extended to Arbroath)
1tp2h Dundee - Arbroath
Plus, some of the Anniesland services
That is the full complement of Central Belt services run by the 158/170 fleet, and, crucially, are worked by units from Haymarket and Eastfield (some of the 158s stable at Corkerhill, but that's not important). It amounts to about 32 diagrams south of the Perth - Arbroath line at any given time, although some units do transfer back and forward from Aberdeen/Inverness to the Central Belt on passenger services, which should be eliminated with the 222 fleet. The longest distance currently run off the wires are the Dundee and Perth services, which conveniently will find themselves running under the wires for about half of the route, following the commissioning of the wires around Glenrothes. Units which out-stable do so at Tweedbank (soon to be wired), Leven (soon to be wired), Perth Depot and Dundee (to be wired).
The highest priority for Scotrail is to be able to bin off the suburban fleet from Haymarket and Eastfield depots. Space there is severely limited, especially at Haymarket, and switching the Anniesland, Fife Circle, Tweedbank, Perth and Dundee services over to the BEMUs would allow you a uniform fleet. Following the commission of all planned electrification, none of these services would spend more than two hours, or about 120 miles off the wires (Dunblane to Arbroath and back), less when Dundee to Arbroath is wired. That allows an internal cascade, with the 158/170s transferring to Aberdeen and bolstering services, replacing the worst 156s on the G&SW routes, and the 158s potentially (although gauging issues may need to be addressed) on the West Highland