A more thorough explanation:

A full-crew law is a regulation that determines the minimum number of employees needed to operate a train or airplane. This law is in place to ensure the safety of passengers and crew members.

• In the United States, the Federal Railroad Administration requires a minimum of two crew members on a freight train. This includes a conductor and an engineer.
• The Federal Aviation Administration requires a minimum of two pilots in the cockpit of a commercial airplane. This includes a captain and a first officer.

These examples illustrate how the full-crew law is applied in different transportation industries. By having a minimum number of crew members, there is a better chance of detecting and preventing accidents or errors that could endanger passengers and crew members.