Railroad spikes are low carbon steel mild steel, approximately 15 points carbon max, even the ones marked HC which stands for higher carbon, not High Carbon only have about 30 points of carbon. The accepted standard for knife steel is at least 60 to 65 points carbon minimum. Email me I have about 20 pages in MS Word on various types of junkyard steel, railroad steel and various steels that are commonly used in knifemaking that Iwill be glad to share with you. On the other hand old leaf and coil springs are good knife steel.
Woody