Most spikes are 5" or 5.5" long. If you can get some of the crud wirebrushed off it would help, also should be iron so a magnet should stick to it, assuming you can get more of the crud off it, or swipe off a clean spot with a file to get closer to the base metal instead of solid corrosion.
The shape makes sense, and given the location I'd also bet it's a RR spike. Who knows maybe there was a siding in that area at one time? I've alos wondered on how to ID old RR sidings and spurs, as I bet you could find some good old stuff in those areas, maybe even silver or RR badges, things like that.