Jet Engines on Rails: UIC 631 & The Era of Gas Turbine Traction

What is UIC 631?

UIC 631 is the International Union of Railways (UIC) leaflet titled “Rules for gas turbines for traction.” Published during the experimental peak of high-speed rail in the 1970s, this standard defines the safety, construction, and operational requirements for using gas turbine engines (similar to jet engines) as the primary power source for locomotives and railcars.

While modern railways are dominated by electric and diesel traction, UIC 631 remains the technical reference for the “Turbo-train” era (such as the early French TGV 001 or the American UAC TurboTrain). It specifies how these high-power, lightweight engines must be integrated into a rolling stock environment, addressing unique challenges like extreme exhaust heat, high-pitch noise, and volatile fuel handling.

Key Technical Requirements

Gas turbines offer an incredible power-to-weight ratio compared to diesel engines, but they introduce significant risks. UIC 631 establishes protocols to mitigate these:

• Overspeed Protection: Unlike diesel engines, a turbine can disintegrate if it spins too fast. The leaflet mandates redundant shutdown systems that cut fuel immediately if RPM limits are exceeded.
• Exhaust Management: Turbine exhaust is extremely hot and voluminous. UIC 631 requires specific ducting designs to prevent the exhaust from melting overhead wires (catenary) or burning passengers on platforms.
• Fuel Safety: Turbines often use aviation-grade kerosene or light diesel. The standard aligns with UIC 623 (diesel engines) but adds stricter controls for leak detection and fire suppression due to the high operating temperatures.

Operational Challenges & Legacy

Although gas turbines allowed trains to reach speeds of 250+ km/h on non-electrified tracks, they fell out of favor due to the 1973 oil crisis and high fuel consumption at idle. UIC 631 stands as a testament to this engineering ambition, ensuring that any remaining or heritage gas-turbine vehicles operate under a unified safety framework.

Comparison: Gas Turbine (UIC 631) vs. Diesel Engine (UIC 623)

Why put a jet engine in a train? The comparison below highlights the trade-offs defined by these standards.