I mean, it's been discussed to death. The legacy Green Line system is riddled with century old tunnels that have curves that are much tighter than those on any other system bar Toronto's. Off the shelf, I believe the Urbos 3 can do 60ft curves? I believe the current standard is 66ft/20m, which is common in places like Portland or Kansas City. The T has stated that they see the Park St loop as non-negotiable for operations, so once the pending Lake/Reservoir Yard retrofits are complete, that 47ft curve is the new ruling curve on the system. The extra 5ft post lechmere is what gives the T enough flexibility for the new type 10, which is likely mechanically a relatively lightly modded Urbos, but its still tight enough to not be off-the-shelf compatible.
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Frankly, for the given length, the type 10 has more segments than the standard Urbos 3 - the type 10 is ~115 ft long, and has 7 segments. The standard Urbos 3 used in Sydney is ~110ft long over 5 segments. Given that it has smaller individual segments that segmentation likely drives fewer windows in of itself, since each segment still needs structural corners. Even in that KC streetcar example, you can see a couple of feet at the flex points where the frameless window is faking it.