Generally I believe it's 1035 steel; might have a HC stamped on the head = high carbon. Here's something I found... might help you: AISI 1035 is a Standard grade Carbon Steel. It is composed of (in weight percentage) 0.32-0.38% Carbon (C), 0.60-0.90% Manganese (Mn), 0.04%(max) Phosphorus (P), 0.05%(max) Sulfur (S), and the base metal Iron (Fe). Other designations of AISI 1035 carbon steel include UNS G10350 and AISI 1035.
Gary is pretty close to dead on. There are dozens, maybe hundreds of different companies making thousands of different spikes. Generally, they are lower carbon content and can be hardened by quenching in water or superquench. Superquench is a home made water based quench that is faster than plain water. I haven't seen the recipe for it for some time. On corners and curves in a track, the RR's used higher carbon spikes, like 1060 range. These usually have a C or HC stamped into them somewhere. These make a passable knife.
My shop in NY was literly in the service yard for the Rail Road. There was a pile of spikes the size of a truck the I could use. The northeast was a mix of dozens of spur lines that dated back to the 1800's, many had been abandoned and torn up. We had great piles of wood, rails, plates and spikes. Much was scrapped. The spikes come in many variaties, from pig steel to high carbon quality.
Look for WHC or HC. These tested out in the 1050-1065 range, made fair knives. The real value being the artistic nature of the spike itself, way cool, made a bunch.
Step one is to sand blast of get all scale off and then check for cracks. A cracked spike will result in a cracked knife. We use ferric chloride to check.
Twist the handle area at the head, then fuller out. There is a lot of steel there.