1. All overnight trains require reservations. A few have seats which you can reserve for 5 to 15 euros. On most, the cheapest accommodation is a 6 berth couchettes which reserves for 20 to 30 euros. Reservations aren’t covered by the railpass, so you will not be sleeping for “free.” You can stay in a cheap hostel for roughly the same cost. Taking local trains to avoid reservations might be an option on a few nights, but will involve several connections and layovers.
2. I couldn’t pull off 30 consecutive nights. Look at the overnight train schedules. A trip that takes 5 or 6 hours on a day train takes 8 or 9 hours on an overnight train. One reason for this is intentional scheduling to have 8 hours of sleep time. The other is that freight trains have the priority at night and you will be stopped on sidings. I wake up every time the train either starts or stops. There are a variety of other reasons that you won’t ever get a great night’s sleep and won’t even get a decent night’s sleep on many nights.
3. I’m sure there are free or cheaper showers around, but that’s going to be difficult. There are a few major cities where there are shower facilities (the only specific I know of is near Gare du Nord in Paris). I think it’s about 5 euros for a shower at the place in Paris.
4. Why would you want to do this? You’ll just be making “I saw the Eiffel Tower” notches in your belt (since you can’t afford the T-shirt). You will not be experiencing Europe. If this is really what you want to do, check out this thread: http://www.eurotrip…. — which actually sounds kind of fun to me as a challenge, not as the way to be a tourist or to travel on the cheap.