Which type of signal cannot be keyed by the train operator?

The home signal is the type of signal that cannot be keyed by the train operator. This signal typically indicates the position of a train relative to a block and is meant to convey critical information about the state of the track ahead. It is primarily used to control train movements at stations and junctions, where the implications of a signal change can affect multiple trains and potentially lead to unsafe situations.

Keying signals refers to the ability of the train operator to manually change or activate them, which is generally reserved for signals that are less critical than home signals. The other types, such as stop signals, yellow warning signals, and caution signals, can often be keyed by the operator as part of their responsibility to manage train operations and safety. The operation of signaling systems is designed to prevent unauthorized or inadvertent changes to crucial aspects of train control, thus maintaining high safety standards in railway operations.