This is the "B" end of BC-11. 

Train brakes operate using air pressure.  Air pressure is applied to release the brakes.  If the system loses air pressure, the brakes "fail safe" and engage to stop the train.

The hose below the coupler connects the air brakes of one car to another.  The loss of air brake pressure in one car will halt the entire train.

In addition to air brakes, some RDCs have engine retarders which help slow the train during normal operation.