Cities: Skylines

Bus station's unlock pretty early on (For me was population 2400). Once you have JUST 1 bus station, you can place as many lines as you want. The lines are found in the same menu as the bus station. The lines do not have to connect 2 stations and even do not have to go anywhere near the 1 station. Press on a random place in the road to start a line, then make 1 stop elsewhere, then click back on the original spot to complete the line. Repeat for a total of 20 lines to get this achievement.

I have a solution for this achievement that is extremely quick, and will also unlock City in Motion 2 (Have 50 transport lines). You would need at least $325,000 to be able to do this.

First, save your current game, and then turn off auto-save if you have it on. Then you will want to construct a road away from your city that is fairly long. Build 5 bus stations along the road you just built. In the same tab where you bought the bus stations from, scroll all the way to the end, and click Bus Line. Face your line of bus stations from the side that is facing the road. You will now click one of the up arrows, click the road in-front of the station, and then click that same arrow again. Do this for every single arrow in the 5 stations, and this achievement will pop.

Now exit your game (so you don't have to actually spend the money), start again, and turn auto-save back on if you want it on.