a little weight can help, bobbers are indeed notoriously light, and often a problem will be pointed out by the last car in a train.
When I started, I did not understand trackwork, switches, flangeways, etc. So I was running a 13 car train and the caboose kept derailing... well, it MUST have been the caboose, took it off, and now the last car derailed... hmm, now THAT car is bad, take if off...
When the train got down to 3 cars, I realized the problem was trackwork, not the cars... good lesson for me.
Greg