When transmitting numbers, what must be done with single digit numbers?

When transmitting numbers, single-digit numbers must be stated and spelled out to ensure clarity and avoid any potential misunderstandings. This practice is essential in communication within the rail industry, especially when precision is critical. Spelling each single-digit number eliminates the chance of miscommunication that could arise from similar-sounding digits.

For instance, saying "one" rather than just stating "1" means there is no ambiguity in how the number is interpreted or received by the listener. This requirement is particularly important in environments where safety and accuracy are paramount, such as rail operations.

Other options suggest simply stating the digit or using formats like alphabetical order, which do not provide the necessary clarity and could lead to confusion in communications. Therefore, the correct protocol is to always state and spell out single-digit numbers.