Assumed I am in a train or car (only co-driver!) with high speeds. Can I catch Pokémon anyway although my environment passes very quickly?
Yes this is possible, after tapping the Pokémon, your movement speed does not affect the catching period.
I have been catching them during hi-speed train travels myself without any problems so far.
Since the last update, Pokémon spawn rates during high speed travel have been reduced significantly. This to prevent people from playing and driving. The limit seems to be somewhere between 60 and 70 Km/h. During travels between 15 Km/h and 60 Km/h a reduced catch rate is observed as well, in combination with a higher escape rate.
TL;DR: It is still possible to catch Pokémon while moving fast, up till certain speeds, and with reduced efficiency.