We are traveling from the US to Spain in April and we want to reserve some train tickets (the high-speed train from Madrid to Barcelona -- not crossing any borders). The online site, loco2.com, requires passport numbers for the tickets. However, my son's passport is being renewed, and we won't have his new number for another couple of weeks. If we book the ticket under his previous passport number, are there likely to be any issues?
(Edit: Going through the site again suggests that it was a misunderstanding, and they do not in fact require passports for tickets.)