Ryanair flies to Berlin. Even before the new airport, they were flying to Schoenefeld which was just outside the city limits. In fact, the old airport was just across the runaways of the new one and you can still reach the old terminal building with the commuter train. Schoenefeld was the old East Berlin airport, which was indeed suboptimal for the load of travellers it got over time.

The are going to renovate Schönefeld and make it a private airport for German politicians. I guess the project will be ready in 20-30 years...

In general, European air travel is often pretty cheap relative to trains. Varies enormously though.

A pilot friend who flew for ryanair showed me around control tower of Frankfurt Hahn (hint: nowhere near the real frankfurt) some ten years ago. We were shooting the shit with the controllers as a private jet plane came in on the radio, asking permission to land. After some back and forth they discovered this was the fake frankfurt, and eventually didn't come in for landing. That got a good chuckle from everyone...