Video shows the buckling swivel thing on the new trolley that jammed up the Green Line after the Marathon

buckling swivel thing
Articulated section? Articulation?

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Articulation involves placing flexible joints and a rolling floor plate at the center of the streetcar so that the streetcar, with its long wheelbase, can maneuver sharp curves. Any longer vehicle has trouble turning along sharp curves, and a streetcar is no exception—longer buses require articulation as well. With articulation, the vehicle is eased along the curve, making sure that it does not drift over into neighboring lanes or hit nearby objects while it turns.