A train must not start unless what condition is met?

The proper signal is crucial for a train to start its journey, as it indicates that it is safe to proceed. In railway operations, signals are essential for controlling train movement, ensuring safety, and coordinating the flow of traffic on the rail network. If a train were to start without a proper signal, it could lead to dangerous situations, such as collisions with other trains or unauthorized movement in track areas where work is being performed.

While having a full crew, warming up an engine, and completing inspections are also important aspects of train readiness and safety, they do not hold as critical a position as the signal in terms of authorizing the train to start. The signal reflects real-time conditions and communications from central control, making it a fundamental requirement for safe train operation.