Playing with 300 2.8 with stacked 2x & 1.4x TC's.
Total focal length 840mm. (EXIF only shows 600mm as only the first TC is reported).
The train is 2.98km away, the control tower & aircraft are 9.87km away. Taken from the deck at my house, distances courtesy of The Photographer's Ephemeris. All images are full frame from Canon 30D, no cropping.