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.
There is, because your own projects always get a priority. Plus, the institutional history. If the same group of people projected your metro for the last 40 years, they carry a lot of local know-how in their heads.
Having a single buyer gives the government the ability to force standardization across cities, and squeeze contractors. Knowing the US though, it’ll become a jobs program for bumfuck towns like Plattsburgh NY
I'm not sure I see any mechanism through which government would gain those powers. I'm also not sure standardization across cities is a good idea - that sounds good but it would slow innovation in technology.