Cait Sith, The rattling that you hear from the tag axle is when the bushings start to go bad. The problem is that because the TA tries to be cheap and stretch out the service schedule on the MCIs. You rarely if ever hear the bushings rattling of NJTs MCIs which to be honest, live much rougher lives.
As for the route variations, its more to spead out the passenger loads during rush hours. Take the 166, you'll have a 166T to Leonia, 166X to Bergenfield and a 166X to Dumont all leave within a couple minutes of each other, each taking the passengers to their respective parts of the line.
Rt.9 is its own animal due to those Park and Rides and the general ridership on Rt.9 itself. The branches (130,132,136) were always 139 trips they just did it to give those folks a quick way to ID what schedule is coming. Keeping the 139 desgination and adding "T,X,Ps" would not have worked as well. You gotta keep in mind that there are variations of the 130 and 132 within those designations.