No, RE stands for 'Regional Express'. There is RE between Hamburg and Bargteheide for example, but Berlin is too far away. REs are rather for commuters from nearby towns.
I have been learning languages on Duolingo and Babbel for years now. None of them I can speak well, but certainly well enough to have a good base of knowledge to start with. Duolingo has the advantage that you can do it whenever you want, while commuting, quickly a lesson before lunch, or on the loo. It just sums up if you consequently keep doing it.
I totally agree with your point, though, that some languages are better suited for Duolingo style of learning than others. For example with Russian it was quite difficult without any grammar basics explained so I needed to additionally read about it, while with Danish it was much easier to grasp the mechanics. Not quite so with the pronunciation...