The New Haven Line running in to NYC goes underground south of 125th St station, beneath Park Avenue into GCS. The clearances inside the long "tunnel" have always been a problem to using large equipment, basically the reason the commuter cars cannot be double-decked. Diesel cannot be run inside the tunnel due to ventilation problems, hence the third-rail shoes. There are three spurs that run off the New Haven Main, one from Norwalk to Danbury,(the rail continues further North, possibly to Pittsfield MASS), one from Bridgeport to Waterbury (the actual line does continue up into Mass., but there is no passenger service beyond Waterbury), and another spur from New Haven up to Springfield, Mass. There are no overhead electrics on either the Waterbury or Springfield lines, so you need the dual-source power if you want to run the trains into GCS. Today, the Waterbury and Springfield trains require pax to deboard and cross over the platform to an electric trainset for the run into NYC. Pax can go either via Amtrak to Penn Station on the West side, or via Metro North commuter trains to GCS in the Center-East side of Manhattan. On the Waterbury line, it is a two-car trainset with an FL9 (I think) pulling. That particular engine is in a state of constant disrepair, so Metro North simply hires buses to provide sub service when it is out of action. The Springfield Line has scheduled Amtrak diesel service, but the latest development was that the State of Conn. obtained some 13 old Budd cars, hammered them together, and started a DMU run on that line, in April 08. At this point all 13 units are apparently conked out, and out of service. At least they tried.