Although in the past the rails had to have a gap between them to cope with temperature expansion/contraction, this technique is no longer applied to high-speed trains. The rails are welded together for several kilometers, and then heated to a high temperature to fully expand. Rail fasteners use very high clamping force, and so the contraction is much less. A second technique is the expansion joint, which may be different: it is no longer a mere gap, but a smooth joint where the two rails are side by side and when they grow or shrink in length they can slide next to each other.