The hardest/most annoying part of this achievement is the fact that you will frequently find that this is seemingly impossible to do due to tunnel setup. In reality, though, you can totally do this. The real trick to this achievement is to use one tunnel to cross the river, then switch it back and forth as necessary between cities on the far side to keep clearing them out. Once enough cities spawn on the far side of the river, it is generally possible to connect them all to each other, allowing you to finally just settle down on one connection.
While it requires a lot of active work on your part, it isn't a terribly difficult achievement due to the low passenger requirements.