In the late 1990s, the Portland shops started rebuilding class B motors with safety cabs; The older motors weren’t such a high priority because they were more likely candidates for retirement/remanufacturing after 70+ years of service, but eventually the class B2 motors started rolling through the shops, and by 2019 most of the surviving units (with the singular exception of class leader #243) had been rebuilt.

The electrical systems inside the locomotive were mainly left as-is (the only serious modification was putting a large transformer & rectifier into the B unit so the BCC4s could operate on the D&H South electrification); the old DC electrical gear was untouched because 4000HP isn’t anything to sneeze at, plus an already depreciated drivetrain is a lot cheaper than installing a modern AC one.

In 2020, when the last two class B1s switchers came out of service, one (#239) was rebuilt with a class N drivetrain inside a BCC4 body.
It has considerably more horsepower than the DC drive BCC4s, so has been classified as class BCC5.

20 of the BCC4s are in operation on the eastern electrification, 4 BCC4s are in service on the South Shore Line, and the sole BCC5 is in operation on the LT&L’s newly-electrified Montréal ↔ St. Johns mainline.