It's not just a coupler and a brake line between a couple of Metra bi-level cars... it's lots, lots more!
Thanks to Hoyt
for summarizing what's here: You have the automatic brake hose plus three more air hoses... one for the Reservoir... one for Actuation (releases the engine brakes w/o altering the automatic brake set)... and one for the Engine brake cylinder. Then you have the MU cable and head end power. In order to operate in push pull mode, the cab car has all the controls of the engine replicated at the end of the train. Therefore, all the same hoses and cables used for connecting engines in multiple must be connected the whole length of the train. Although I'm not familiar with passenger cars from a railroader's perspective, there is also some type of cable (air or electrical I don't know) that replicates the door controls between the cars as well, so the conductor need only operate one set of doors and that command input will be cloned on all cars set to do so.