Should we buy our train tickets before we leave Canada..when we arrive in Nuremberg we are visiting friends in Cochem.would it be cheaper to purchase tickets early
Generally speaking, you can usually save money by buying individual train tickets ahead for longer distance, high-speed trains in many countries, such as Germany, purchased directly through that countries official Rail website (www.bahn.com for Germany). Most of these advance purchase tickets are non-refundable, non-exchangeable, so you should be ready to commit at time of purchase. If you want greater flexibility, you can almost always buy individual tickets a day or two ahead, right at the station, but for a higher price. The D-Bahn site will show both full-fare and discounted "savings fares" for German trains.
If you are doing a significant amount of rail travel, especially in Germany, than a German railpass could save you money over full-fare tickets as well as give you greater flexability. It really depends on how much train travel you plan on doing and when. You really have to do the math to see what is better for your particular trip. Right now, the German railpass is on sale for travel between November 1st and January 31. See this webpage for details: http://ricksteves.raileurope.com/rail-tickets-passes/german-rail-pass/index.html