IMO, if you are using a rail line that follows a common bicycle trail such as the Katy Trail (Missouri River Runner Line) or GAP / C& O (Capitol Limited Line), it's a good idea to consider booking months in advance, because they will often have an insanely limited number of bike spots available on that train (as little as 4 or 6 bikes per train) and often you can get shut out if you haven't made your reservation months in advance. Less a problem if you're solo but if it is my wife and I or a small group, we've often had to alter our riding dates because we have tried to get our bikes on, even weeks ahead during popular touring times.

And here often is what will shut you out. I want to ride from St. Louis to Kansas City 250 miles, but they won't allow my bike on because just yesterday some bike commuter booked for just 9 miles outside of Kansas City in, and only at that point will I become bike number 5 after 241 miles. So frustrating, especially when that person eventually cancels but you can't know that in advance and when you get there, you see that the train could easily handle twice the number of bikes that they allow.

I know I stated that each train experience can be more or less accommodating than another, but I have never seen a time when they will break the limit number of bikes allowed on a train for even a few miles.