Cities: Skylines

so this didnt take very long for me

my city size was 50k but i just put the helicopter depot in which is under the transport hub
then you place a few stops around which is a different building under the transport hub


then i had the free public transport policy active and it literally took about 5 minutes to do


not sure if its bugged or i genuinely did 15000 people but i did have literally thousands waiting at each stop


here is a video guide so you can see :)

You can approach this in the same way as all of your other transit related achievements. Set up a helicopter depot, set some helicopter stops around the perimeter of your city, create a path between all your stops, create a route hitting each stop, and give it some time. My city had about 300k population, and I'd say it took about 45 minutes to hit 15k passengers.