It could be done, I think that was the plan if the recently replaced BL cars had proven up to snuff... The Blue Line's narrower, but I imagine that could be dealt with too.
My understanding though is that the current plan is to replace the OL fleet and the 1500-1600-1700 Red Line fleet at the same time, and thus have a single bigger with two different-sized bodies on top of identical trains... The 1500-1600's are ~10 years older than the Orange Line cars, though they've been rehabbed once.
I'm not sure Siemens could really just fire up the 0700 line again... I doubt they leave those lying around for long.
It wouldn't be that hard. Heavy rail cars are very generic. The Blue Line's supposed uniqueness (short cars and dual third rail + pantographs) aren't very unique at all. The Hawker Siddeley 0600's and 01200's were part of a combined order. They are the same exact car.
-- The length difference is length of carbody only...they're the exact same width and have the exact same components. The trucks and electrical guts underneath are just spread 16+ feet longer under the floor on Orange. But it's the exact same guts. The Blue cars could've been retrofitted with the same ATO signal boxes too (the OL cars were delivered without them...that was added to them in 1987 when the SW Corridor opened with cab signals). The factory can do different lengths of the same carbody pretty easily, since a lot of legacy systems (including NYC and London) have different dimensions on different lines.
-- The couple inches difference in platform heights is adjustable by tuning the shocks to raise or lower the car. All of them have a hydraulic air "ballast" under the floor for ride quality and so things don't settle over time and get out-of-alignment with the platform. If you've ever ridden on a random Blue/Orange/Red car that doesn't quite line up with the platform edge that's usually because the air ballast needs a recalibration or (for too-low cars) has sprung a leak.
-- The power source is the same 600 volt DC from above or below--on all 4 lines + Mattapan--so it doesn't matter whether a pantograph or third rail takes it in. The OL 01200's have the exact same roof mounts and pantograph wiring as the retired BL 0600's. Bring an 01200 to Orient Heights, install an old 0600 pantograph lying around in the carhouse in 45 minutes' labor, and it can run on the above-ground portion as-is, raise and lower the pantograph with the same pushbutton, etc. You can even do this on the Red Line, although those cars were not delivered with roof plug-ins. You can even run them on trolley poles. Seashore Trolley Museum in Maine takes its collection of ex-T heavy rail cars out on makeshift poles...including the Blue 0600's and every single type of Red Line car they have dating back to when the line opened. (To answer that question...yes, you could grab a set of Red 01800's and take it to Mattapan today if one were modded with a pole.)
All of this is why the T had plans to send 24 of the 0600's through a midlife overhaul and send them to Orange to trainline with the 01200's, with the 12's likewise getting rebuilt. Would give them all another dozen years of service and get them to 40 years old (like the Red 01500/01600's) before retirement. Pantographs would come off, ATO signal box put in, floor height ballast adjusted, and blue stripe repainted to orange. They wouldn't be segregated at all...most OL consists would have a 'stubby' shorter 0600 sandwiched in, with the tradeoff in losing ~40 seats per 6-car train offset by much denser headways.
Unfortunately they could not come up with 24 cars in decent enough shape to send through the rebuild program. The carbody corrosion from the salt air was much worse than they thought, and the extra body work required upped costs too high. It proved better value to just replace the OL fleet whole, combine the order with the Red Line, and shoot for cars that would last 25 years before first rebuild instead of 10-15 on extended lifespan.
So if Siemens restarted production of the 0700's without a single modification except for car length and the ATO signal box, they can pump out the Orange and Red fleets pretty quickly. All it takes is restarting their factory capacity. The design phase would be pretty fast if the T can refrain from overcustomization. And there's not much to overcustomize with heavy rail, nor has the T done too much of that with past orders (Green and commuter rail, on the other hand. . .). The 0600's/01200's are based on the same car order as PATH's recently retired
PA3's, and NYC Subway's small fleet of
R110B's are based on the Red 01800's and were tacked on at the end of the T's production order 18 years ago. We could easily have 3 lines running the same model.
Since the 0700's are operating well they are probably the favored vendor for the next contract, and the T is amply motivated to standardize on one model because it makes maintaining the fleet hugely easier. This just happens to be the first time a Red order has happened in the same decade as one of the others, so it's the first opportunity they've ever had to sync the fleets and reduce their maintenance overhead a bit. And the only reason it hasn't been procured yet is budget. It's a humongous number of cars...by far the largest they've ever placed at once. I seriously think for expediting it the state might want to consider a one-time bond just to get it the hell over with. They can't put this off any longer for the Legislature to fiddle while the T burns.
This is why it is not cost-effective for the T to modify the clearances in the tunnels to use identical cars. There's absolutely no difference in potential makes, and the tincan put on top is by far the cheapest part of the unit cost. If the lines don't interconnect at all (and really, there is no obvious way of getting Blue and Orange linked without some crazy convuluted routing that doesn't fit any established commute pattern) there's no compelling reason to do radical surgery on the East Boston tunnel or run whole trains of much smaller, lower-capacity cars through the Washington St. tunnel for purely Transit OCD purposes.
