Because of the union of Aragon and Castile to form Spain its railroad with an event called "Iberian Wedding".
You and I have a very different definition of railroad. An event that doesn't always happen due to conditions not being right and having a chance that the AI choose a different option (It's like 10% chance of rejecting the union, right? I know I just saw this recently on a Let's Play.) seems to miss the definition of a "railroaded event."
Anyways, so long as the AI has a moderate chance (5-20%) of choosing something different in an event like the Iberian Wedding I think things would be fine. Maybe that chance should be given throughout all more deterministic events with the chances of deviating from the more historical choice being greater depending on certain factors and time from campaign start.