Orange would be the easiest to convert to Red dimensions. The tunnel from Chinatown to Haymarket--4 close-packed stations--is the only remaining original installation. The 1975-built Haymarket-north tunnel is built to RL dimensions, and so is the 1967-built South Cove tunnel. All tunnels built post-1912, even on Green, are at RL dimensions except for a couple pinch points (Copley Jct. curve, the low-ceiling C/D portal) where they pinched pennies on a few tougher feet of tunneling. Surface stations are pretty much moot, since all you have to change are the platform juts. Install temporary ones to OL dimensions before making the switch, then yank them off that day of the changeover to reveal the RL dimension juts. It can switch literally overnight. The cars themselves have secondary shocks that raise or lower a few inches (Blue and Orange cars are designed with that fresh from the factory in case Blue cars ever need to get redeployed on Orange).
Now, what's the justification for shaving the walls back in the Washington St. tunnel to widen the clearances? I can't think of a single Red-Orange connection possibility that would make that worthwhile or serve a clear-cut need. That's why no one ever attempted it even dating back to the BERy days (where they did, of course convert Blue from trolleys and did have 50-year plans to convert all Green tunnels to Blue-sized rapid transit).
It's definitely too expensive to do for sole reason of equipment standardization's sake. The equipment already is standardized. Orange and Blue have 100% operationally and parts-identical cars for 30 years, fitted into different-size carbodies. The 01200's and now-retired 0600's can trainline with each other unmodified if Blue got the same cab signal units that were added to the 01200's after the El closed. They almost did send 24 of the 0600's to Orange in lieu of a new Orange order, opting to rehab all Orange cars and using the 24 rehabbed extras as an expansion fleet. It didn't work--the 0600 carbodies were too rusted out to rebuild--but had they proceeded you would be riding mixed consists today comprised of short + long cars running on much beefier headways. The 01200's also have the same pantograph mounts on the roofs. Had the Reading extension ever been done they would've had pantos installed later...slap one on, plug it in, and you're running under overhead.
If Siemens wins the next contract for the combo Orange+Red order, they will quite literally re-fire up the factory with the 0700 blueprints and pump out a few hundred of the same Blue Line guts underneath an Orange or Red carbody. And you could set up a demo consist on the Wellington test track for the ribbon-cutting ceremony with a Blue Line 0700, an Orange Line 01400, and a Red Line 01900 trainlining seamlessly with each other. Same guts, same parts, different tincan clothing. That's it. No reason to unify them if the service patterns are end-to-end instead of anywhere snaking to anywhere like the NYC subway. If they need more service patterns, that's what branching is for and that's what the whole light rail mode is for.
