The problem of moving ground
The Trans-Siberian Railroad stretches 9,288 kilometers across Russia, with large sections built on ground that is permanently frozen, known as permafrost. This frozen soil, which covers about 65% of Russian territory, can be up to 1,500 meters deep and has remained frozen for thousands of years. Building on this landscape is a difficult engineering problem.
Permafrost is not uniformly solid. The upper "active layer" thaws in the summer and refreezes in the winter. This annual cycle causes the ground to expand and contract. When water in the soil freezes, it expands, pushing the ground surface upward in a process called frost heave. This can deform and misalign railway tracks, creating an unsafe surface. Conversely, heat from the railway's dark ballast and the friction of the trains can thaw the ice within the permafrost, turning the once-solid ground into unstable mud and causing the tracks to sink. Early sections of the railroad built at the turn of the 20th century suffered from significant deformations almost immediately after construction due to these thermal changes.
A passive cooling solution
To keep the ground beneath the tracks reliably frozen, engineers use a passive heat-transfer technology called the two-phase closed thermosyphon. A thermosyphon is a sealed metal tube containing a pressurized working fluid, such as carbon dioxide or ammonia, that has a very low boiling point. The bottom of the tube, the evaporator section, is buried deep in the ground, while the top, the condenser section, is exposed to the air, often with radiator fins to increase its surface area.
During the frigid Siberian winter, the cold air chills the condenser section of the pipe. The gas inside condenses into a liquid and flows by gravity to the warmer evaporator section below ground. This super-cooled liquid absorbs heat from the surrounding soil, causing the liquid to boil and turn back into a gas. The gas then rises to the condenser, where it releases its heat into the cold winter air, and the cycle repeats. This process continuously and passively pumps heat out of the ground all winter, creating a frozen, stable foundation that can remain solid through the summer. Because the system relies on the air being colder than the ground, it automatically becomes dormant during warmer months. This technology allows the railroad to operate on permafrost that would otherwise be too unstable to support the massive loads of freight and passenger trains.