In the Snes9x, cheat menu, set the following HEX address to the maximum number of cities/routes you wish to open in the game. In the later game scenarios, these are set to the same by design so you don't interconnect you hubs increasing your revenue exponentially.
I like max cities 60 and max routes 75. Granted you can only achieve those numbers playing Scenario 4, Supersonic Travel, years 2000-2020, and you can hit the design maximums easily around 2008 if you take Tokyo as your home and charge your citizens way too much.
7E2E41 <- the hex address to max cities 7E3F67 <- the hex address to max routes