Can I take the train from Tegel to Berlin? All I see talked about is a bus.
The bus called "TXL" is the express bus that runs frequently between the busy spots in Berlin and Tegel airport. But TXL isn't the only bus serving Tegel airport and may not be the most convenient bus to your destination in Berlin. You can use Google maps to find the best transit routing to your destination - could be a different bus than TXL or may be a bus connection to a U-bahn or S-bahn train.
Also, be warned that there are both U-Bahn and rail stations with the word "Tegel" in them, but they are not conveniently linked to the airport.