Actually railroad spikes contain so little carbon that the quenching and tempering is pointless.

Mike blue researched it and eventually tracked down an engineer from a company that made them. Even the 'high carbon ' ones contain only .2 % which is perfect for spikes that you don't want to risk fatigue hardening.

You'll find plenty of people to argue about this but the fact is that the amount of carbon is so low it will not harden properly