# Why Don’t Railroads Need Expansion Joints? Channel: Veritasium Source: https://www.youtube.com/watch?v=Rdj5-6t6QI8 Recap page: https://rapidrecap.app/video/Rdj5-6t6QI8 Generated: 2025-07-07T05:17:59.853+00:00 --- ## Quick Overview **Key Takeaway:** Railroads do not require expansion joints because their continuous welded rails are engineered to withstand extreme thermal stresses by being held in a constant state of tension or compression, preventing buckling or separation. **Key Points:** - Railroads do not need expansion joints because they are designed to be in a state of constant stress, either in tension or compression, which prevents buckling or pulling apart. - Continuous Welded Rail (CWR) is installed at a 'neutral temperature' where it experiences no thermal stress, typically around 90 degrees Fahrenheit. - When temperatures deviate from the neutral temperature, the rail experiences significant forces, but these forces are contained within the rail itself due to its immense stiffness and the surrounding ballast. - The rail's resistance to buckling is significantly increased by the lateral resistance provided by the ties and ballast, which are designed to hold the rail firmly in place. ## Summary Railroads do not use expansion joints because their tracks are constructed with Continuous Welded Rail (CWR), a system designed to manage thermal expansion and contraction through internal stress rather than physical movement. When CWR is laid, it is done so at a specific 'neutral temperature,' typically around 90 degrees Fahrenheit, at which point the rail experiences no thermal stress. As temperatures rise above this neutral point, the rail attempts to expand, but it is rigidly held in place by the ties and ballast, causing immense compressive forces to build up within the rail. Conversely, when temperatures drop below the neutral temperature, tensile forces develop as the rail attempts to contract. These internal forces, which can amount to hundreds of thousands of pounds over a mile of track, are contained by the sheer stiffness of the steel rail and the significant lateral resistance provided by the surrounding track structure. The ties and ballast are crucial in preventing the rail from buckling under compression or pulling apart under tension. This method is a more robust and maintenance-efficient solution than incorporating numerous expansion joints, which would introduce weak points and require constant upkeep across the vast lengths of railway lines. The design ensures the track remains stable and functional under a wide range of environmental conditions by keeping the rail in a controlled state of tension or compression. **Key Points:** - Railroads employ Continuous Welded Rail (CWR) which is designed to be under constant internal stress, eliminating the need for expansion joints. - CWR is installed at a 'neutral temperature,' typically around 90 degrees Fahrenheit, where it experiences no thermal stress. - Temperature deviations from the neutral point induce significant compressive or tensile forces within the rail, which are absorbed internally. - The ties and ballast provide crucial lateral resistance, preventing the rail from buckling under compression or pulling apart under tension. - The high stiffness of steel rail allows it to withstand hundreds of thousands of pounds of force per mile due to thermal changes. - Traditional expansion joints are impractical for railroads due to the scale of the forces and the maintenance challenges they would present over long distances. **Context:** Railroad tracks, specifically those utilizing Continuous Welded Rail (CWR), are engineered to manage thermal expansion and contraction without the need for traditional expansion joints. Unlike bridges or buildings where joints accommodate movement, railway tracks are designed to be in a constant state of internal stress. This design leverages the inherent stiffness of steel and the robust support provided by the ties and ballast to contain the immense forces generated by temperature fluctuations, ensuring track stability and integrity across vast distances. ## Detailed Analysis The fundamental reason railroads do not need expansion joints is their use of Continuous Welded Rail (CWR), which is installed to be under constant stress rather than allowing for thermal movement. When CWR is laid, it is done so at a specific 'neutral temperature,' typically around 90 degrees Fahrenheit, where the rail experiences no thermal stress. As the ambient temperature deviates from this neutral temperature, the rail attempts to expand or contract. However, because it is rigidly held in place by the ties and ballast, it cannot move freely. Instead, internal forces build up within the rail: compressive forces when the temperature rises above the neutral temperature, and tensile forces when it drops below. These forces are immense, reaching hundreds of thousands of pounds over a mile of track. The track structure, particularly the ballast and ties, provides crucial lateral resistance that prevents the rail from buckling under compressive forces or pulling apart under tensile forces. The rail's high stiffness means it can withstand significant internal stress before deforming. The design ensures that the rail is always in a state of either tension or compression, which is a controlled and predictable condition. This method is far more effective and less maintenance-intensive than trying to accommodate thermal expansion with joints, which would introduce weak points and require frequent inspection and repair across thousands of miles of track. The video demonstrates that the forces involved are so substantial that traditional expansion joints would be impractical and less reliable than the current CWR system. **Key Moments:** - **01:20**: Visual explanation of how continuous welded rail (CWR) is laid and how it handles thermal expansion and contraction without joints. ![Screenshot at 01:20](https://ss.rapidrecap.app/screens/Rdj5-6t6QI8/00-01-20.png) - **02:45**: Demonstration or animation showing the forces at play when rail expands or contracts and how the ballast and ties resist these forces. ![Screenshot at 02:45](https://ss.rapidrecap.app/screens/Rdj5-6t6QI8/00-02-45.png) - **03:30**: Explanation of the 'neutral temperature' concept and its importance in CWR installation. ![Screenshot at 03:30](https://ss.rapidrecap.app/screens/Rdj5-6t6QI8/00-03-30.png) - **04:50**: Comparison of rail expansion with other materials like concrete or steel beams in bridges, highlighting the unique challenges and solutions for railroads. ![Screenshot at 04:50](https://ss.rapidrecap.app/screens/Rdj5-6t6QI8/00-04-50.png) - **06:15**: Visual of a buckled track section, illustrating what happens when the system fails to contain thermal stresses, emphasizing the importance of proper design and maintenance. ![Screenshot at 06:15](https://ss.rapidrecap.app/screens/Rdj5-6t6QI8/00-06-15.png) **Insights:** - The concept of 'constant stress' is central to CWR design, allowing railroads to absorb thermal expansion and contraction internally rather than through physical movement. - The track's lateral resistance, provided by ties and ballast, is as critical as the rail's material properties in preventing thermal buckling. - The neutral temperature for CWR installation is a critical parameter, directly influencing the magnitude of thermal stresses the rail will experience. - Traditional expansion joints, while common in other infrastructure, are fundamentally unsuited for the scale and forces involved in railway tracks. **Action Items:** - Ensure proper ballast and tie integrity to maintain the lateral resistance crucial for preventing rail buckling. - Monitor rail temperatures and track conditions, especially during extreme weather, to identify potential stress points or weaknesses. - Adhere to established neutral temperature installation guidelines for CWR to minimize excessive thermal stresses.