Rail joints caused wear and tear, partly on passing locomotives and wagons due to vibrations (the clunking sound), and partly on the track itself when the wheels hammered against the rails at the joint.

Rail joints were a way to accommodate the expansion of materials that occurred during hot weather. Too long rail sections without joints could lead to sun kinks.

Today, the track is welded together along its entire length. Sun kinks are prevented by ensuring that the track is heavy and well anchored in the sleepers, which in turn are held in place by coarse and heavy ballast, such as macadam. The track simply cannot move. Thus, the stresses caused by material expansion are absorbed by the rails themselves.

Below is equipment for Thermite welding from the 1960s. Thermite welding was already used in Germany in the early 1900s but was introduced to SJ in 1952 after successful trials on the Nässjö-Solberga stretch.

Thermite welding works as follows: The ends of the rails are heated up to 900 - 1,000°C using gas. A mixture of aluminum powder, iron oxide, and some alloys is placed in a crucible above the joint. The mixture is ignited with a special thermite igniter, which works somewhat like a sparkler. The igniter reaches approximately 1,200 degrees. The mixture in the crucible ignites, and in less than half a minute, the temperature rises to 2,000 - 3,000°C. The molten steel collects at the bottom, and slag forms on top.

Now, the molten metal is allowed to flow down into a mold around the joint itself. This results in a casting where the two rail ends and the joint itself become one. The strength becomes very high. Once the joint has solidified somewhat, the mold is removed. Excess material is chiseled away while it is still hot and soft. The joint is then ground to be perfectly smooth.