Which of the following is NOT a key type used in metro elevator systems?

In metro elevator systems, the types of keys are specifically designed to facilitate different levels of access for authorized personnel. The service master key, metro master key, and maintenance key all serve important functions in managing access and ensuring the efficient operation and servicing of elevator systems.

The service master key typically allows for access to all service areas and control panels within the metro elevator system, allowing for essential operational tasks to be performed. Metro master keys are designed for broader access across different systems or facilities in the metro network, while maintenance keys provide authorized maintenance personnel with the ability to access machinery for repairs and routine checks.

On the other hand, the standard key is not a recognized type of key used in metro elevator systems. While it may serve various functions in other contexts or types of buildings, it does not have a specific designation of role or authorization in relation to the particular requirements and functions of metro elevator systems. Therefore, it stands out as the option that is not categorized within the key types utilized in this specific context.