Permanent Way (history) - Continuous Welded Track

Continuous Welded Track

Continuous welded track was developed in response to the observation that the bulk of track maintenance work takes place at the joints. As steel production and manufacturing processes improved, the rail lengths installed were progressively increased, and the logical extension of this would be to eliminate the joints altogether.

A major obstacle to doing so is thermal expansion: the rails expand in higher temperatures. Without joints, there is no room for the rails to expand; as the rails get warmer they will develop an enormous force in trying to expand. If prevented from expanding, they develop a force of 1.7 tonnes (17 kN) for every 1 degree Celsius of temperature change in a practical rail section.

If a small cube of metal is compressed between the jaws of a press, it will contract—that is it will be squashed somewhat—and a very large force can be resisted by it without ultimate failure. However if a long piece of metal of the same cross section is compressed, it will deform sideways into a bow shape; the process is called buckling, and the compressive force it can withstand is very much less.

If the long thin piece of metal could be constrained to prevent it from buckling (e.g. by being contained inside a tube) then it can resist a much higher compressive force. If the rails can be constrained in a similar way, they can be prevented from buckling. The weight of the track resists buckling upwards, so buckling is most likely to take place laterally. This is prevented by:

• providing heavy sleepers, that generate friction on the ballast bed
• ensuring that the sleepers are well supported on consolidated ballast to enable the generation of the friction
• providing consolidated ballast around the sides of the sleepers to provide additional friction
• heating the rails when they are installed and fastened in cool or cold weather, so that the expansion on the hottest days is less than otherwise
• making sure that any rail added if rail breaks during cold weather is removed before warm weather returns.
• making sure that curves do not line themselves inward during cold weather sufficiently to make buckling more likely when warm weather returns
• taking precautions when track maintenance work is performed in hot weather, and making sure ballast is sufficiently consolidated before full-speed operation is resumed.

If the rail is held so that it cannot expand at all, then there is no limit on the length of rail that can be handled. The expansive force in a one-foot length of rail at a certain temperature is the same as in a 100-mile length of rail. Early continuous welded rail was installed in limited lengths only because of technological limitations. However at the end of the cwr section where it abutted older, ordinary jointed track, that track would be unable to resist the expansive force and the jointed track might be forced to buckle. To prevent that, special expansion switches, sometimes called breathers, were installed. The expansion switches could accommodate a considerable expansive movement—typically four inches (100mm) or so—in the end section of the cwr without passing the movement on to the jointed track.

The cwr is installed and fastened down at an optimum temperature, to ensure that the highest possible expansive force is limited. This temperature is called the stress-free temperature, and in the UK it is 27 °C (81 °F). It is in the upper range of ordinary outdoor temperatures, and the actual installation work tends to be done at cooler temperatures. Originally the rails were physically heated to the stress-free temperature with propane gas heaters; they were then rattled with hand bars to eliminate any binding, preventing even expansion, and then clipped down. Since about 1963 however hydraulic jacks are used to physically stretch the rails while they are supported on temporary rollers. By stretching the rails to the length they would be if they were at the stress-free temperature, then there is no need to heat them; they can just be clipped down before the jacks are released.

The cwr rails are made by welding ordinary rails together. For many years rails could only be made in lengths of up to 60 ft (18 288mm) in Britain, and the factory welding process made them into 600, 900 or 1200 ft lengths, depending on the factory. The process used was a flash-butt process in which high electrical currents are used to soften the rail end, and the ends are then forced together by rams. The flash-butt process is very reliable, providing that the factory ensured good geometry of the rail ends.

The long rails could be conveyed to site by special train, and unloaded on to the ground (by chaining the end in position and pulling the train out from underneath the rails). The long rails had to be welded together (or to adjacent track) using a site welding process, and after initial experimentation the proprietary Thermit welding process was used. This was an alumino-thermic process in which a powder 'portion' was ignited; the aluminium was the fuel and a metallurgically appropriate composition of molten steel descended into the gap between the rail ends, contained in refractory moulds.

The original SmW process was very sensitive to operator skill, and as the welding was usually the final process before returning the track to traffic, time pressure was sometimes applied resulting in unwanted improper welds. The improved SkV process was less sensitive and over the years weld quality improved.

It is worth mentioning that jointed track has suffered buckles in the past; the fish-plates need to be removed and greased annually (the requirement was relaxed to bi-annually in 1993) and where this was forgotten or where ballast conditions were especially weak, buckling took place in hot weather. In addition, if rails were allowed to creep, it was always possible that several successive joints closed up, so that the expansion gap was lost, with inevitable results at the onset of hot weather.