When compromising welds on rail ends of two different rail sections, the preheating time shall be for the time required for the ___________ rail's section.

In the context of thermite welding, particularly when working with rail ends of differing sizes or sections, the principle behind preheating is to ensure uniform heat distribution and adequate fusion during the welding process. When two different rail sections are to be welded together, the preheating time is determined by the characteristics of the larger rail's section.

The larger rail section requires a longer preheating time because it has more mass and, consequently, more energy must be supplied to raise its temperature to a suitable level for effective welding. This ensures that when the thermite reaction takes place, both rail ends are adequately heated, promoting proper alloy bonding and minimizing issues such as cracking or insufficient fusion.

This principle is crucial in rail welding since a well-executed weld is essential for maintaining the integrity and safety of rail operations. The choice of using the larger section's specifications for preheating is a standard practice among welders to achieve the best results in thermite welding.