We are the Suzerain of a city state, meaning we should be able to control everything that City can or will do for the foreseeable future, right? So please allow us to build improvements or let us control what they can build in certain economic terms such as Railways, Canals etc. Being a Suzerain of something means we control pretty much everything, historically, in this case a City State right down to foreign, economic, and even religious policy. Heck allow us to trade with other civs to build economic features for them, or even help them, with Military Engineers to build or even hasten the building of railways etc. It can and should even affect Diplomacy for the better or even worse depending on the relations.