During the additional walk-around checks on a set of doubles, what position should the converter dolly's air tank drain valve be in?

Explanation

The converter dolly's air tank drain valve, also called its petcock, must be closed. The dolly's tank has to hold pressure for the dolly's brakes to work, so a drain valve left open bleeds away the air those brakes depend on; that is a separate point from the maintenance reason for draining tanks in the first place, which is clearing the water and compressor oil that collect in them. This same petcock is what a driver deliberately opens to release the dolly's brakes while wheeling it into position during coupling, which is exactly why it is easy to leave open and worth a specific check afterward. 49 CFR 383.111(a)(17)(iii) names coupling system defects as a required inspection knowledge area without listing this item, so the check is attributed to the manual. The wrong options all misread what the tank does: draining while driving would stop the brakes holding pressure at all, the dolly is not supplied from the trailer ahead in place of its own tank, and that tank serves the dolly's service brakes too, so the valve position is not optional.

Source: 49 CFR 383.111(a)(17)(iii)