Cities: Skylines

If you followed the walkthrough available for this game, you'll already have trams set up tracing your whole city. I did this achievement late in the game, and it popped quickly as I was converting tram routes to trollybus routes. I only needed to replace two tram loops with trollybus tracks, and the achievement popped while I was in the middle of creating my third trollybus loop.

My suggestion:
Create a save, swap out your tram depot for a trollybus depot, convert a single loop of tram tracks to trollybus tracks (you will have to delete the streets and re-create them, you cannot use the upgrade tool unless you change the streets to a third type of street first and then convert that to trollybus tracks), create a route following the same path as the tram route you replaced, wait it out, and reload your save once your achievement pops.

this achievement is pretty simple
just got to go to the transport hub , go to the second option along which is the trolleybus section
from here you place down a depot and upgrade a few roads in a loop around important areas in your city , i personally looped it between my residential areas and industrial areas
once you have created the lines you also want to go to your policies and turn on the free public transport policy because this will make more people use your trolleybus service

obviously the bigger the city the faster you will get this , but its just a waiting game

anyways here is a video to help anyone out