Yellow air line?

It's not air it's electric.

 

Mines greeeen

 

Its an auxiliary electrical pigtail to send power to the dump valve on the rear trailer axle.

 

And extra lights or functions where applicable

 

I have seen a third air line on a few stretch trailers.
Similar to a air assisted sliding tandem to open and close pins.

 

Most of the stretches I've been around used the red gladhand hooked up to a different gladhand to release the slide pins.

 

I've plumbed in a manual valve on those so I don't have to switch the line.

 

I've seen stretches with valves too, I shouldn't of spoke in absolutes. But the flats I pulled you switched the gladhand.