The land behind my house is criss crossed with remnants of narrow gauge railroad lines that were used to log the land. The logs were loaded onto the flat cars and taken to the sawmills. The spikes that I find are considerably smaller than regular railroad spikes. From research, the last line was torn out in the 1910's at the latest. You might have an old logging or mining camp nearby as most were made near the tracks, at least in my area.
Doug