Europe in December

"How busy are trains and hostels that time of year? Will we need to have reservations?" With a railpass, you usually need to purchase a compulsory reservation anyway. But would you have needed a reservation to find a seat if you didn't have the pass? Probably not, unless you travel on some of the faster trains during commuting hours.

In December, concentrate more on cities than rural site-seeing. Daylight hours are very short, and morning are often foggy and overcast. This doesn't really affect what you can see and do in the cities, which are adequately illuminated, but it does in the countryside.

I normally stick to the local (regional) trains because I stop in small towns instead of city to city trips.

In December the local trains are crowded during rush hours, other than that it should be easy to find a seat and no reservation needed.

Hostels should be pretty easy to stay without reservations in December.

A couple of exceptions I've run into:

When a convention is in town, hostels can fill up just like other hotels. We found a very full hostel in Verona because there was a cheese convention going on. We got beds but had to stay in separate dorms.

Some hostels are always popular. Burg Stahleck, a castle turned hostel in Bacharach, seems to be full all the time. I've only been able to stay be getting a reservation well in advance.

As Tom said the days are short in December. Make sure you're mentally prepared for that - it can be a shock when you come from somewhere the days don't change much seasonally.

Try and get to some of the Christmas markets in the German cities and towns, even the smaller towns will have one.