Which line cannot have an articulated bus run under the 66th Street underpass?

The M66 line cannot operate articulated buses under the 66th Street underpass due to specific clearance limitations and restrictions related to the dimensions of the underpass. Articulated buses, which are longer and have a pivoting joint, require greater height and width clearance than standard buses. In areas where infrastructure elements like underpasses exist, ensuring that all vehicles can safely pass is critical, and in this case, the M66's route requires a bus design that adheres to the height restrictions of that passage.

On the other hand, the M34, M15, and BX12 lines have been established to accommodate articulated buses, meaning their routes and the underpasses they traverse have the necessary clearance to allow these longer vehicles to operate safely. This highlights the importance of infrastructure compatibility with vehicle design in urban transportation planning.