I was thinking of using regular sand for gravel on the rails since the size is very close to scale ie smaller than ones scale hand,. Here is what I would do First sift it to get all the rubbish out then wave a magnet over it too get any fillings etc, out , third put it in a salt - spice shaker after it has been washed and dried.
spray a mixture of white glue and isopropel alcohol on the sand when you place it on the rails. Spray paint with a light gray since this simulates oil grease etc comming off the cars on the gravel .
any suggestions to improve this readily welcome