I was curious about this as well.
I have a fairly flexible and loose trip across Europe planned, I will land in Holland and will catch a cheap flight from Kiev a couple of weeks later, the only thing that will be set in stone is my departure date from Kiev. The path I take and the cities I stop in are completely up in the air.
Can I rely on booking train tickets a few days in advance or should I try and have a more cement plan in advance?