The imaginary city that was the destination of the train in the holodeck fantasy created by the Enterprise's emerging intelligence, standing for the vertion particles it needed.
Vertiform City is spoken of throughout the Next Generation episode Emergence. When the ship's systems begin to form a mind of their own, its thoughts are played out on the holodeck as a blend of running programmes centred on the Orient Express, with a conductor, an engineer, a gunslinger, a knight and other figures all anxious that the train should reach the city.
The crew work out that the name points to vertion particles, which the new intelligence needs to complete a life form it is building in a cargo bay. The ship takes itself to a white dwarf star to gather them, and the train stops at Keystone City along the way.
When the natural supply runs out, La Forge creates vertions artificially in a nearby nebula. On the holodeck the train pulls in at New Vertiform City, and the completed entity departs into space.