Terry,
Every abandoned grade is different. Lots of factors. How close to a town is the grade? Years of kids picking up and throwing track bolts and nuts, or anything throwable can clean a site. Sometimes it depends on who the scrapper was. Some grades, like the Manns Creek, have so much hardware that it seems only the rails themselves were taken. Even the time the line was scrapped can matter. Lines scrapped during ww2 can be pretty clean, because that scrap was needed for the war effort. How close was a mill to process the scrap, or a buyer? Here in Pittsburgh, I trace the Pittsburgh Southern, in operation from 1879 to 1886. With lots of mlls and a ready market for scrap, two world wars and a depression, metal artifacts can be non existent. Sometimes a little chunk of coal, or a "clinker" from a firebox is the only way I know I've found the grade. To make matters worse, the PSRR used a slag as fill that sets off my metal detector, so you learn to ignore some hits after digging up nothing but slag. But its a great feeling when you do find something, proof that these wonderful little railroads really did exist.
brian b