Some of the ridiculous-sounding Rail Sales are actually 70%, even 80 or 90% off the regular rate. At that price, it's a heck of a lot cheaper than the 50% off child fares, or the 10% off AAA or NARP rates.
Over this past summer, my son and I each rode the entire route of the Pere Marquette (Grand Rapids MI to Chicago IL) for $4 and change a piece on a Rail Sale deal.
It is definitely worthwhile if the Rail Sale is offered for only a piece of a route to take the Rail Sale for that segment and then book at the best rate for the balance. There is nothing wrong with splitting a ticket up if it is financially advantageous to you. As an example, lately the Pennsylvanian and Three Rivers are coming up dirt cheap from Pittsburgh to Chicago. If you were going from the east coast to Chicago, you could book this, and then find another rate for the portion of your trip between the east coast and Pittsburgh. On a trip to Cleveland last summer, I also split my ticket on the Lake Shore Limited: New York to Buffalo, and Buffalo to Cleveland.