According to the website specs all of the Bora 50 bags are to big, by volume, to meet carryon specs United (45.4L) or Lufthansa (44L). The small one (46L) might squeak thru, but the middle sized one is 10% over and the large (54L) one is 20% too big. Plus the overall shape appears too long vs the other dimensions (United limit total of all dimensins to 45 in.). Also, at 4-5#, in my opinion, it weighs too much as a carry-on (mine weighs 1-3/4#).
It looks like a good bag for all day backpacking, but a bit of overkill for just "traveling from trains to hostels".