How many possible express routes are there across the entire MBTA system?
I know someone on reddit posted about Harvard to Alewife express trains on the Red Line.
On the Green Line E branch Brigham - Northeastern can be used for express, and it seems East Somerville - Medford is another segment used for express.
Not sure about the B, C, or D branches. Although I'd assume Harvard Ave (B), Coolidge Corner (C), and Riverside and Reservior (D) are used as start or endpoints for express.
Does the Orange and Blue lines never use express trains? Are there any other station pairs on the Red Line used for express?
The rule is we can now only be only expressed between ADA-accessible stations. This doesn't matter that much on the heavy rail with the one inaccessible stop being Bowdoin, which you couldn't express past no matter how much you try (at least till Red-Blue is done then *every* train will express past the current station site, but I digress); however, this matters quite a bit on the Green.
Coolidge Corner and Washington Square, then Washington to Cleveland Circle, are extremely common on the C - they will even often give it as one command "Coolidge to Washington and Washington back". Sometimes you'll see St. Mary's to Coolidge. On the B, a very common one is Harvard Ave to Washington St, then Washington to BC; you also get BU East to Babcock or Harvard a lot, and sometimes they will run express from Kenmore. In the past, as
@HenryAlan noted, Blandford to Packard's Corner used to be common from what I've heard too, but with the newer ADA rules they really aren't supposed to do that anymore.
The D and E give you more options with more accessible stations to work with. Brigham to Northeastern is by far the most common on the E, then East Somerville to Medford. The D is all over the place, but they will basically always stop at Reservoir and most of the time they stop at Woodland; besides that, there are many possible combinations that are used on the D.
Occasionally, they will also express on the inbound -- I got expressed from Riverside to Newton Highlands on Monday.
I've seen Ruggles to Forest Hills trips go express on the Orange, though it isn't common these days as they still are not quite back at pre-covid service levels.
I can't say I've ever heard of it happening on the Blue Line, I'd be interested to know if it's ever happened and what stations they'd even skip -- if it's ever happened, Orient Heights to Wonderland would by far make the most sense.