chain of events

While ultracaps do
store energy, they are typically only sized to maintain an output load
for a period of about 30 seconds in the flywheel bridge. This
application isn't really a UPS per se

Why not? As long as those 30 seconds suffice to e.g. get said generator
online, the system serves to maintin power in the event of a utility power
failure, making it an uninterruptible power supply.

Which part of UPS don't I get?

Anno.

The "U" part.

Uninteruptable means you never miss a cycle, the light bulbs never blink,
etc.

Uninteruptable supplies are now cheap for small systems and available, but
costly, when aplied on a grand scale. What you descrbed is a backup or
standby generator with automatic changeover.

If you think of the situation that the passengers might see inside the
terminal, most of the lighting and possibly the schedule monitors would be
backed up by the standby generator--because there are enough battery powered
emergency lights to prevent panic and/or injuries. OTOH, the computers
which drive the monitors would be on UPS. Today, most of the monitors would
probably be on UPS as well, since it is no longer expensive to do so--and
probably contributes to calm amoung the public in the terminal.

Peter