Railroad spikes are made of steel, not pure iron, and are designed to secure the rail to the railroad tie or sleeper. These fasteners prevent the rails from spreading apart and maintain the correct distance, or gauge, between the tracks. The spike must withstand constant heavy forces from passing trains to ensure the stability and safety of the entire railway system.
The Direct Answer Steel Versus Iron
The primary material for modern railroad spikes is carbon steel, an alloy of iron and a small percentage of carbon. This composition fundamentally differs from pure iron, which is too soft, and cast iron, which is too brittle for this application. Steel’s properties are engineered through precise control of the carbon content, usually falling into the low-to-medium carbon range for railroad spikes. This range is selected for its mechanical properties.
The carbon element distinguishes steel from iron, acting as a hardening agent within the iron matrix. Pure iron, such as wrought iron, is highly malleable but lacks the necessary strength for railway use. Conversely, cast iron has a high carbon content, making it extremely hard but prone to shattering under impact. The low-to-medium carbon steel used in spikes, typically containing between 0.25% to 0.6% carbon, achieves a balanced combination of strength and flexibility.
This specific carbon content allows the spike to be tough enough to resist bending yet ductile enough to absorb shock without fracturing. The chemical recipe is calibrated to prevent the spike from snapping when it is driven into the wood or composite tie.
Why Steel is Essential for Railroad Spikes
Steel provides the resilience needed to resist the complex forces exerted by heavy, fast-moving trains. A spike must endure significant shear forces, which are sideways pressures that try to push the rail away from the tie. These lateral forces are particularly intense on curved sections of track where the train’s wheels press against the rail. The spike requires high ultimate tensile strength to resist being pulled out of the tie.
The spike must also withstand continuous impact and bending stress from the vertical wheel load that presses directly over the tie plate. This constant loading and unloading causes bending strain, which can lead to fatigue failure over time. The slight ductility of carbon steel allows it to bend slightly without permanent deformation under normal loads. This property is a necessary compromise for the high-impact environment.
The Historical Context of Spike Materials
The question about iron arises because the earliest railroad spikes were made from wrought iron. This material was the most common and accessible metal for fasteners in the early 19th century. These early spikes were often hand-forged and were sufficient for the lower speeds and lighter loads of the first railway systems.
A significant material shift occurred as rail technology advanced and the demand for stronger components increased. The widespread adoption of processes like the Bessemer process in the late 19th century made steel significantly more affordable and easier to mass-produce. Steel was rapidly adopted because its superior strength and resistance to wear made the tracks safer and reduced the frequency of maintenance. Carbon steel became the standard for all modern railroad spikes.