A rail is an informal unit of length. Generally the length of a rail is 39 feet, but other lengths such as 33 feet or 30 feet might have been the rail length in use when the benchmark was set. Some modern rails are continuous and hence have no particular length, so the length of the rails used at the time the benchmark was set will have to be based on guesswork.
A possible search method is to use 39, 33, and 30 feet at the same time, searching at the distances indicated by using each of them.
A successful mark recovery should re-state the distance from rails to feet.