Jumping into this thread, and updating my master list of tenders, I feel like I can shed some light.
Going for a coffee with people I know at TS, there are a couple of interesting points brought up. From these conversations, I have a much better idea of what their desire is at this point, however, obviously, plans may change when manufacturers come back with a bid.
EMUs
Firstly, there is a general wish for this fleet to be broadly inter-compatible, uniform and of one single modular design. There will not be separate suburban and metro style units, instead carriage lengths, seating layouts and door placements will be the same. The desire is for a standard 23/24m carriage length with doors at 1/4 and 3/4 (TS and Scotrail have been incredibly pleased by the capacity of the 385s to soak up passengers, especially on the E&G routes). They will be high acceleration, 100mph capable designs, although depending on acceleration profiles, 90mph max wouldn't be the end of the world.
Its likely that most, if not all, of the pure EMUs will be a uniform fleet of 5 cars. These would be the ones intended for low-level routes through the Argyll and North Clyde lines, the Cathcart Circle and the Glasgow Southside lines. Five car length, walk through and level boarding EMUs will fundamentally transform the passenger experience, however would also mean that ticket inspectors get to travel the whole train, which has been a long desire for Scotrail on the Glasgow urban network. You lose some length going for 5x23m, however you save capacity from the cabs in the middle, 5x24m would be a net increase in capacity from 2x3x20m, again due to the cabs in the middle. The whole network is set up as a 6x20m railway, so you don't have to much around with platform lengths anywhere.
Some of the EMUs may end up being three cars, depending on whether or not shuffling the 280s off the Glasgow Southside routes allows you to fully turn Inverclyde over to the Desiros, and whether or not the 334s can be sweated beyond 2035. The 380s are already 23.5m length cars, and so Inverclyde can take 6x23m. The 334s are not, and going for a uniform fleet would mean you'd need to find alternative stabling space, but that's neither here nor there.
Basically, the fleet of EMUs sees off everything that isn't a 380, 385, and perhaps the 334s. They would be based at Shields, Yoker, Corkerhill and Bathgate.
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.
TS wants a uniform fleet of high capacity, suburban layout, doors at 1/4 and 3/4, 3x23/24m units. This actually increases capacity (3/6x23m) versus the current set up, where units run as any combination of 2/3/4/5/6 cars.
*I think I'm going to start using BEMUs (battery electric multiple unit) as opposed to BMUs (bi-mode multiple unit) as a stylistic decision, just to try and standardise what it is that we are actually talking about, as bi-mode can mean a plethora of different things depending on the context.
