When driving a set of doubles, which shut-off valve must be closed in the last trailer?

When driving a set of doubles, it is essential to close the shut-off valve in the last trailer to ensure safety and prevent any potential hazards associated with cargo. By closing the back valve in the last trailer, you effectively isolate that trailer's systems from the rest of the combination. This is crucial because if there were a leak or malfunction in the last trailer, it could pose a significant risk to the operation of the entire set.

When the back valve is closed, it prevents any unintended movement or spillage from that trailer, allowing for safer handling of the cargo being transported. Closing the back valve also helps maintain control over the situation if an issue arises, as the last trailer won't affect the entire combination.

In contrast, closing either the forward valve or side valves would not provide the same level of safety and control specific to the last trailer in a doubles setup. Therefore, the requirement to close the back valve is a vital part of ensuring the integrity and safety of the transportation process.