You'll learn about balanced routes early in the story mode when they become available. This can also be accomplished in career mode straight away.
To do this, you need to create a route and use the 'Demand' filter to add at least 10 stops, ensuring a balance between the blue +1 and the purple -1 demand values. You don't need to drive the route, and the route doesn't have to be particularly efficient, as long as it has at least 10 stops and the demand is balanced on both sides. Once the trophy pops, you can delete the route if you don't wish to keep it.