The backpack I used hiking in the UK doing the Coast to Coast on four separate trips of about 50 miles a trip was either a 75 or 90 litre. It weighed about 40 lbs when packed. After the first two trips I used Packhorse and they drove my backpack to my next accommodation.
A backpack is less secure than a suitcase in transportation so would suggest getting a fanny pack to keep your valuables in while you are on the aircraft and on a train.
Yes I remember arriving late by train when I was 16 enroute to a job in France. Arrived at Dijon France at one in the morning. Got a cab and arrived at the Chateau(think of French Fawlty Towers) at 3 am.
Enjoy your trip to Germany. The last trip my daughter and her husband did to Ireland they lost her suitcase. Our daughter went out and bought what she needed to get by. Suitcase was delivered three days later to their hotels. Airline compensated her.
I think next time if I do hiking in the UK a 50 litre backpack would be easier. I like cargo pants as well. T shirts and basics...
