I do not beleive that a cross tie (very course open grain) would offer enough resistance to mushroom the head of a spike.
There for I can't see any reason for high Carbon. Besides the low Carbon would weather better.
Carbon makes little yellow streamers that burst open like a tiny explosion.
But what is the point? You plan to tensil test it? You can weld it with 6010, 7018 or 12018 or you can weld it with SS.
There is not enough mass in a Rail Road spike to justify much time and effort in preheating it. I bet the entire spike will get pretty hot throught out befor you finish welding.
Pre heat for the most part is to prevent the mass of the base material from quenching the weld metal to quickly. and most likely you plan to weld it to low Carbon angle iron frame work.
I have made a couple of devises for the same purpose a few years back, even made a drag using old Cotton picker spindles, Now there is some high Carbon material.