Did the diplodocus dinosaur travel in herds?

Yes. However, we have only found evidence that juveniles and adults were part of these herds. The hatchlings were probably left to fend for themselves until they grew large enough to keep up with a herd, and also to reach the same food. Some evidence suggests that young sauropods, including diplodocus, ate different foods then the adults. If this is so, it would be very inefficient and detrimental for all of them to travel together because they would need to go to different places for the young the adults to get their food.