Let's see. Cologne to Berlin takes ~4:40 hours by train. Flixbus takes 9-10 hours, not counting the time it takes to get to their departure station which would involve a train journey as it's not actually in the city centre.
Flixbus is 50€ cheaper when traveling that route tomorrow but that's about all it has going for it.
Flixbus is 50€ cheaper when traveling that route tomorrow but that's about all it has going for it.