Worried About Shaken Baby Syndrome

Tonight my family was at a fall festival and my 7 week twin girls were in backpack carriers. We went on a children's train ride pulled by a tractor. I had no idea how fast and bumpy that ride would be! My husband and I both held their heads to keep them from getting too jostled, but it was an extremely bumpy ride and now I'm worried about them. I've read about the symptoms and haven't seen anything yet but should I really be worried about this or are they better off since their heads were secure? I don't want anything to happen to my girls. :(