Why T cars are not interchangeable between lines

The Boston subway has it's quirks and a big one is that each line has it's own dedicated fleet of cars.

Tom Long in the Northwest Globe edition of Starts and Stops gets the T to explain why

The MBTA was created by the consolidations of private carriers, resulting in three rail lines with different dimensions. The unique tunnel and platform dimensions are the most significant characteristics that prevent a standardized vehicle design.

The length and width of the rail cars are also problematic. The cars range in width from 9 feet 2 inches on the Blue Line to 10 feet on the Red Line. The Red Line vehicles are 69 feet 6 inches long, the Orange Line 65 feet 4 inches, and the Blue Line 48 feet 6 inches.

The reason the Blue Line cars are shorter is because the Bowdoin loop requires a vehicle that can negotiate a 66-foot curve. The shorter cars enable a tighter turning radius for the vehicle. The Orange or Red Line would not be able to negotiate this turning radius in a tunnel.

Pesaturo said the T investigated the possibility of refitting Orange Line cars for use on the Blue Line while waiting for new cars, but the cost was prohibitive.

This website offers a quick overview of the history of the subway in Boston
Of note is what occured on the Easter/Patriot's Day weekend of 1924. The entire East Boston tunnel was converted from trolley to heavy rail subway IN THREE DAYS!!!!

Conversion of East Boston Line to Multiple Unit Trains. The East Boston Line had been built in 1904 for trolleys with no premonition of a conversion to multiple unit operation. Over an April double-holiday weekend, the East Boston Line underwent major change. Patrons went home aboard familiar trolley cars. They returned on Monday on brand-new steel multiple unit trains which they could board and leave at platform level. The grade in the tunnel was 5%, severe for trolleys, but even more so for multiple unit. subway trains. There were sharp curves and close clearances. Nearly four miles of third rail had to be hoisted onto pre-set insulators, electrically connected, and tested.