If the emergency (supply) line between a tractor and trailer is cut, broken, or disconnected during a crash, what happens to the trailer's brakes, and how is the emergency line typically color-coded at the glad hand connection?

AAMVA CDL Manual, Section 6, subsections 6.2.4-6.2.5 (p. 6-5), 'Trailer Air-lines' and 'Hose Couplers (Glad Hands)'; 49 CFR 383.111(b)(3)

Knowing that a severed emergency line causes automatic braking, not a loss of braking, explains why combination vehicles are designed to be safe even in a worst-case separation during a crash. This also matters for everyday coupling: mixing up the red emergency and blue service glad hands can cause real operating problems, which is why the color convention is tested.