It doesn't matter. A standards-compliant ramp is vehicle-agnostic. If it's OK for a tractor-trailer, it's OK for an artic. The current-edition AASHTO standard the Ted ramps were held to is also more rigorous than the 1965 regs of the Pike Extension, so if trucks larger than an articulated bus can take the Newbury merge...they can take the Ted merge.And I have never seen an articulated bus using those ramps, much less a slower dual mode articulated bus. Correct me if I'm wrong but those routes are all 40ft buses?
The only proof that matters is the ramp meeting current AASHTO standards. It appears to do so. Splitting hairs about 40-footer vs. 60-footer buses is utterly, totally irrelevant to the standards. So if MassDOT is claiming the ramp isn't safe enough, there's a reason other than merge length behind it. And they aren't telling us what exactly that reason is.