I took a trip today to the Railroad Museum of Pennsylvania. One of the displays includes a Reading MU commuter coach. What was the purpose of this device mounted at the front of the roof? It appears that this was unique to Reading Company.
The device at the roof ends of the original Reading MU's was a power bus bar contactor. The MU cars had bus bars which ran the length of the cars and were connected to the pantographs and the system allowed multiple cars to draw overhead power through a minimum number of raised pantographs. This was designed to save wear on both the pantographs and the catenary. The bus bar system was removed from the cars prior to the opening of the Center City Commuter Tunnel due to clearance issues and the presence of the phase break between Amtrak / Septa power at Girard Avenue.
The cars can be seen with the bus bars / contactors removed here: http://www.thebluecomet.com/rdgWTrent1.jpg