Sherman's Memoirs talk about destroying rail and also repairing rail. Although it's hard to find those passages from the table of contents.
http://www.sonofthesouth.net/union-generals/sherman/memoirs/general-sherman-memoirs.htm
Shermans army utterly destroyed some railroads by heating the rails and twisting them around trees. This damage was effectively permanent because the south could not replace the rails or unbend them. This method takes a lot of manpower and a lot of time, and you have to destroy many miles because there is extra rail on sidetracks and secondary spurs to replace some losses.
Most rail damage could be quickly repaired. For example the Rebs tried to interfere with Shermans supply trains coming to Atlanta from Chatanooga, but the Feds were able to repair them.
I imagine most rail sabotage was intended to derail the first locomotive that came by. A few men with levers can loosen one end of a track pretty fast. Another method is to use explosives. Explosives might ruin a few sections of rail, but replacements can usually be found nearby on sidetracks and secondary spurs.
Derailment was a major threat. Trains were used very differently in wartime than they were in peace. Typically one engine would only pull 2 or 3 cars in disputed territory because the risks were so great.