There are serious issues with that approach. If you don't have continuous funding from the projects, you end up with a big overhead of highly paid engineers without work for them to do. Then you have to lay them off, so you lose the institutional knowledge anyway.

We tried this early on with sound transit in Washington state, and because engineering work is boom and bust on a project by project basis, the model just doesn't work. The good people left for better jobs, and we were left with a team that basically couldn't produce, leading to massive delays on the next set of projects.

I think it is for this reason that Switzerland has a fixed budget for their railway construction. And it seems to pay off, Swiss railway is exceptional.

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.