I have found earlier posts on using these websites very helpful. I have one further question and some advice of my own.
First, the question: my problem is with Renfe, the website for the Spanish train system. After much trouble, I succeeded in purchasing tickets; I got an email with a locator number but I have no tickets. I cannot figure out how to retrieve them (the website is in Spanish and is not user friendly in general). Are there any tips? For those struggling with the website, in general, I would recommend tripadvisor, which walks you through the process:
http://www.tripadvisor.com/Travel-g187514-c80518/Madrid:Spain:Buying.Renfe.Tickets.Online..html
The information on retrieving tickets has not worked for me, though.
Now some advice about Trenitalia:
I initially had no problem purchasing (and printing) tickets online from trenitalia. Then I was not able to and had basically given up. Today, I succeeded in buying tickets. I would recommend two things: use internet explorer instead of firefox; try different credit cards. If all else fails, it really is simple to buy tickets at the train station, including from the machines, which are very user friendly.