Double headed rails represent a significant and formative stage in the history of railway track development. As one of the earliest rail profiles, their design was based on a logical but ultimately flawed principle of economy and reusability. Though now obsolete and superseded by more advanced profiles, understanding the specifications and characteristics of double headed rails is crucial for comprehending the evolution of modern track engineering. This technical guide Xingrail provides a detailed examination of this historical rail type, focusing on its specific dimensions, material properties, and the fastening systems it required.
Design and Structural Specifications of Double Headed Rails
The defining characteristic of double headed rails is their symmetrical, I-shaped or dumbbell-like cross-section. The profile consists of three main parts: an upper head, a central web, and a lower foot that is identical in shape and size to the upper head.
- Design Principle: The core idea behind this design was reusability. Engineers theorized that once train traffic wore down the top running surface (the head), workers could simply unfasten, flip, and reinstall the rail to present a “new” running surface. They intended this approach to double each rail’s service life and offer significant cost savings in an era when steel production was expensive and labor-intensive.
- Structural Profile: The I-shaped section provided good vertical stiffness, allowing the rail to act as a beam to support heavy locomotive loads and distribute them to the sleepers below. Foundries typically forged the head, web, and foot as a single piece of iron or, later, steel.
Key Dimensions and Proportions
The dimensions of double headed rails varied by manufacturer and railway company, as standardization was not yet widespread. However, they generally followed a set of common proportions.
The weight of these rails was typically measured in pounds per yard. Common weights for mainline double headed sections ranged from 75 lb/yd to 95 lb/yd.
Material Properties and Manufacturing
The era of double headed rails spanned a critical transition in metallurgy, from wrought iron to steel.
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Wrought Iron: The earliest double headed sections were forged from wrought iron. While strong in tension, wrought iron was relatively soft and prone to delamination under the intense, rolling contact stress from train wheels. It wore out quickly, which ironically reinforced the perceived need for a reversible design.
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Bessemer and Open-Hearth Steel: With the invention of the Bessemer process in the 1850s and later the open-hearth process, steel became commercially viable for rail production. Steel offered vastly superior hardness, wear resistance, and homogeneity compared to iron. Steel double headed rails had a much longer primary service life.
The chemical composition was simple by modern standards, consisting mainly of iron with a controlled amount of carbon to provide hardness.
Fastening System: The “Chair”
Unlike modern flat-bottomed rails that can be spiked directly to a sleeper (with a tie plate), the symmetrical design of double headed rails made them unstable. They could not stand upright on their own and required a special fastening system to hold them in place. This system was centered around a heavy cast-iron component called a “chair.”
The Railway Chair
The chair was a robust iron casting designed to cradle the foot of the rail and secure it to a wooden sleeper.
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Design: Each chair had a flat base that was spiked or bolted to the sleeper. The top of the chair featured two “jaws.” Workers seated the rail between these jaws. One jaw was cast to fit snugly against one side of the rail’s web and foot, while the other jaw was offset to leave a gap.
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Function: The chair served two primary functions:
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Support and Gauge Holding: It provided a wide, stable base to support the rail and distribute the load onto the sleeper. By holding the rail firmly, the chairs were responsible for maintaining the correct track gauge.
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Securing the Rail: It provided the means by which the rail was physically fastened to the track structure.
The Wooden Key (Wedge)
The rail was secured into the chair using a wooden wedge, known as a “key” or “quoin.”
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Material and Design: Keys were made from a block of hardwood (often oak) that was compressed and cut into a taper.
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Installation: The key was driven horizontally into the gap between the rail and the offset jaw of the chair. As workers hammered it in, the wedge action created an immense clamping force that pressed the rail firmly against the opposing jaw. On straight track, installers typically placed keys on the outside of the rail to use the train’s lateral forces to help tighten them.
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Maintenance: Wooden keys were a major source of maintenance. They would shrink in dry weather, causing them to loosen, and swell in wet weather. Track maintenance crews (“gangers” or “trackmen”) had to regularly walk the line to hammer keys back into place. Eventually, steel spring keys were developed to provide a more permanent and elastic fastening.
The Flaw in the Design: Why Double Headed Rails Failed
The central premise of the double headed rails—reversibility—proved to be a critical failure in practice. Engineers soon discovered that the lower head, seated in the cast-iron chairs, was not preserved in pristine condition.
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Chair Galling: The immense pressure and slight movements of the rail within the chairs caused significant wear, indentation, and galling on the surface of the lower head. This damage was concentrated at the points where the rail rested on the chair.
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Unsuitability for Reuse: When worn rails were eventually turned over, the damaged lower head provided a rough and uneven running surface. This created a poor ride quality and, more importantly, introduced dynamic impact loads that were detrimental to both the track structure and the rolling stock. The “second life” of the rail performed so poorly that engineers largely abandoned the practice.
This realization led directly to the development of the Bull Head Rail. Engineers recognized that since the bottom head would never be used as a running surface, it did not need to be the same size as the top head. They redesigned the profile with a larger, more durable top head to maximize initial wear life and a smaller, non-functional bottom head that was optimized simply for fitting securely into the railway chair. The bull head rail retained the same chair-and-key fastening system but abandoned the failed concept of reversibility, paving the way for the eventual dominance of the flat-bottomed rail.
