What about renting them out to other cities? There are dozens of cities in Europe alone that are planning extensions of their metro systems, or even building ones from scratch (Cluj-Napoca in Romania, recently. Krakow in Poland soon.)

A morbid equivalent from the Middle Ages: bigger medieval cities had their own headsman, and they solved the risk of underemployment by sending him on external "jobs" to smaller towns where executions were rare.

At that point, you're just hiring an engineering firm, so there's no real benefit to government doing it.