Sir - Robert Tieskens is right to draw attention to cross-border taxation issues (RG4.04 p194).
Another issue is the distortion of competition caused by Value Added Tax. This is payable on rail tickets in Germany and Sweden, and possibly other European Union countries; it is not payable on air tickets. So if you are travelling between Frankfurt and Stuttgart or Stockholm and Malmö you can travel VAT-free by air, or pay VAT on your rail ticket.
The Europe Regions Airlines Association has recently been fulminating somewhat tendentiously against the discrimination which air carriers face when compared to railways; their attention should be drawn to this point. Discrimination is not always against air!
International Air-Rail Organisation