Which part of the welding setup must be safeguarded to prevent shifting during the process?

The correct answer emphasizes the importance of stabilizing the rail ends during thermite welding to ensure the integrity and accuracy of the weld. The rail ends must be securely positioned to prevent any movement, which could lead to misalignment and compromise the quality of the weld. In thermite welding, the molten iron generated by the reaction must flow smoothly into the gap between the rail ends to create a strong bond. If the rail ends shift even slightly during the process, it can result in a poor weld, leading to weaknesses that can affect rail performance and safety.

The other options, while important components of the welding setup, do not directly relate to the specific need for stabilization during the welding process. For example, the welding machine needs to function properly, but its stability does not have the same immediate impact on the weld integrity as the positioning of the rail ends. Similarly, while cooling blankets and igniters are critical safety and operational elements, they are not components that need to be physically secured in the same way the rail ends do during this specific part of the welding operation.