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Soft and High-Carbon Spikes
Railroad spikes are all made of steel but in two grades: grade 1 spikes are "soft," while grade 2 spikes are harder and of high-carbon and require marking with an "HC" symbol.
Cut Spikes
Cut spikes are the most basic type of railway spike, with a square or rectangular head and a single shaft with a square cross-section. Workers hammer them into the wood of the tie and let the resilience of the wood apply a compressive force that holds the spikes in place.
Hairpin Spikes
Hairpin spikes have a rectangular head but have two prongs that drive apart when the workers knock the spike into the wood. The grain of the wood resists the sideways motion of the prongs, providing more compressive force to hold the spike in place. These spikes suffer fatigue as heat, cold, and moisture expand and compress the tie and alter the force applied to the prongs.
Screw Spikes
Screw spikes are driven into the tie by rotation, which jams wood (or concrete, as these are widely used with concrete ties) into the grooves between screw threads. This provides more grip than either hairpin or cut spikes. Screw spikes are vulnerable to being shaken loose as vibration from passing trains slowly unscrews them from the ties.
Self-Securing Spikes
These spikes resemble hairpin spikes in that they have two prongs and are hammered into the tie. They function on different principles, however. When hammered in, the prongs of a self-securing spike split much wider than those of a hairpin spike. These prongs have inside surfaces that are notched to grip the wood between the prongs. The elastic force of the spike pressing inward on the wood between the prongs hold these spikes in, instead of using the compressive force of the wood itself.