Under what condition does a firefighter gain free access to transportation?

A firefighter gains free access to transportation primarily when in full uniform. This condition is based on the principle that being in uniform not only signifies the firefighter's official capacity but also provides a visual identification that communicates their role and the nature of their work. In emergencies or when their services are needed, this recognition allows them to access transportation without impediments, ensuring they can respond rapidly.

The other options, while possibly relevant in other contexts, do not hold the same level of importance in granting access to transportation for firefighters. For instance, being accompanied by an officer may not universally apply and could vary by jurisdiction or situation. During training sessions, while firefighters may receive some form of access, this does not equate to the broader provision of free access. Likewise, possessing a valid ID is necessary for identification but may not automatically imply access to transportation in the same way that being in uniform does.