What is this pneumatic truck part???

[evh]

 

[Bogie1632]

Glad hand adapter. Likely for filling tanks or testing brakes from a shop air source.

Red denotes the emergency brakes. Blue would be service brakes.

V/R
Bogie

 

[Wrench97]

Red is the supply line, air in the supply line releases the parking brakes, air in the blue line applies the service brakes.
What you have there is a glad hand, extension(makes it easier to hook up) and a quick disconnect fitting to hook it to shop air for servicing the trailer.

 

[evh]

Thank you!