This reminds of the project that aimed at creating a new business/commercial complex south-east of Paris in Noisy-Le-Grand. A real estate promoter had a big project, and a metro line was designed, then built, but the real estate project went into bankruptcy and never got out of the ground.[0]
The metro line was completed, inaugurated, but never opened to the public, and eventually mothballed. For quite a while, it was rumored that they operated trains once a month to keep the system working and maintained, not sure up until when.