Well actually most of the Racer's drops are sideways, so that's why the seat dividers are needed (In 1927, this was an amazing feat to have track that twisted). The older trains didn't have seat dividers, and if you go to the Pittsburgh History Museum, you can still see one of the old cars on display.
The Jack Rabbit doesn't need seat dividers. For the double dip, the stationery lap bar does just fine ;).
The Thunderbolt doesn't need seat dividers because one of the best parts of the rides is squishing your partner ... unless your partner squishes you ;).