Practical Engineering - Why Railroads Don't Need Expansion Joints

Short Summary

Main recommendation: Modern railways use continuous welded rail (CWR), welded segments with almost no joints, instead of jointed rail, because jointed rail is noisy and wears out faster. CWR works without expansion gaps because its thermal expansion is restrained, and railroads manage the resulting buckling risk by installing rail at a high neutral temperature.

Key Points

• Jointed rail was a workaround for transport (rail sections too long to move) and thermal expansion (gaps bridged by fish plates let rails grow on hot days), but each joint caused the clickety-clack noise and wear.
• Wheel impacts at joints batter the steel, add load on the ballast, and cause freight cars to rock side to side ("rock and roll"), raising maintenance costs.
• Restrained thermal expansion is the core idea: a material that cannot expand builds up compressive or tensile stress instead, which is how CWR avoids expansion joints.
• The main danger in CWR is compressive stress causing buckling ("sun kink"), which has caused many derailments, so US regulations require detailed installation, adjustment, inspection, and maintenance procedures.
• Railroads avoid compression mostly by setting a high neutral temperature (the temperature at which the rail has zero stress), so rail is rarely hotter than neutral.

Details Worth Noting

• Concrete instructions:
  • Install rail on hot days so the rail is at or near its neutral temperature. If that is not possible, use heaters to raise the rail temperature.
  • If heating is not enough, use hydraulic jacks to induce tensile force in the rails before welding them together.
  • On cold days, when tensile stress is highest, inspect the rails to make sure they have not pulled apart.
  • Use ballast (crushed rock) to hold ties in place and prevent horizontal sliding, which would allow the rail to buckle.
  • Choose a neutral temperature as high as you can safely get away with (the Central Florida Rail Corridor uses a neutral temperature of 105°F / just over 40°C).
• Standard US jointed rail sections were about 39 feet long, laid so gaps just closed on the hottest days.
• Thermal expansion formula: multiply the temperature change by the material's coefficient of thermal expansion. The aluminum rod in the demo expands about 0.002% per °C; steel is about half that.
• Full restraint across a Texas-style range (below freezing to over 100°F) gives about 15,000 PSI (100 MPa) of stress in steel, below steel's strength, but buckling can still occur because long, slender rails are prone to it.
• Buckling load falls off with the square of length: halving the unsupported length roughly quadruples the critical force (demo: about 375 g vs. about 1,500 g).
• At the ends of CWR sections, bridges, or older jointed rail, "breathers" use diagonal tapered joints so wheels transition smoothly while leaving room for thermal movement. Insulated plates electrically isolate track sections for signals and crossings.

Bottom Line

Continuous welded rail is quieter and cheaper to maintain than jointed rail, and it works because engineers control stress and buckling through neutral temperature, ballast restraint, and careful installation and inspection rather than by allowing expansion.