Why the new high speed train track can be connected without cracks for temperature variations?

Question

The new high-speed trains can travel up to 350KM/hr. The train tracks are continuously connected without cracks . This is why it can reach high speed. How can it adjust to the variations because of the expansion/contraction due to temperature variations?

Answer 1

I wondered about that myself, so I looked it up on wikipedia. Basically they are welded in place while hot, and the only problem is cold weather -- and then they just stretch.

Continuous welded rail

Most modern railways use continuous welded rail (CWR), sometimes referred to as ribbon rails. In this form of track, the rails are welded together by utilising flash butt welding to form one continuous rail that may be several kilometres long, or thermite welding to repair or splice together existing CWR segments. Because there are few joints, this form of track is very strong, gives a smooth ride, and needs less maintenance. The first welded track was used in Germany in 1924 and the US in 1930 and has become common on main lines since the 1950s.

Flash butt welding is the preferred process which involves an automated track laying machine running a strong electrical current through the touching ends of two unjoined pieces of rail. The ends become white hot due to electrical resistance and are then pressed together forming a strong weld. Thermite welding is a manual process requiring a reaction crucible and form to contain the molten iron. Thermite-bonded joints are also seen as less reliable and more prone to fracture or break.

Because of the increased strength of welded track, trains can travel on it at higher speeds and with less friction. Welded rails are more expensive to lay than jointed tracks, but have much lower maintenance costs.

Rails expand in hot weather and shrink in cold weather. Because welded track has very few expansion joints, if no special measures are taken it could become distorted in hot weather and cause a derailment (a condition known in North America as sun kink, and in Britain as buckling). In North America a rail broken due to cold-related contraction is known as a pull-apart.

To avoid this, welded rails are laid on concrete or steel sleepers, which are so heavy they hold the rails firmly in place. Great attention is paid to compacting the ballast effectively, particularly the shoulder over the ends of the sleepers, to prevent them from moving. Even so, in extreme weather, foot patrols monitor sections of track known to be problematic.

After new segments of rail are laid, or defective rails replaced (welded in), the rails are artificially stressed. The stressing process involves either heating the rails causing them to expand, or stretching the rails with hydraulic equipment. They are then fastened (clipped) to the sleepers in their expanded form. This process ensures that the rail will not expand much further in subsequent hot weather. In cold weather the rails try to contract, but because they are firmly fastened, cannot do so. In effect, stressed rails are a bit like a piece of stretched elastic firmly fastened down.

Engineers try to heat the rail to a temperature roughly midway between the average extremes of hot and cold (this is known as the 'rail neutral temperature'). If temperatures reach outside normal ranges however, welded rail can buckle in a hotter than usual summer or can actually break in a colder than anticipated winter. In North America, because broken rails are typically detected by the signaling system; they are seen as less of a problem than heat kinks which are not detected. For this reason, and because it is harder to break a rail than displace the trackbed, CWR is usually installed at a temperature of 90 °F (32 °C), to cope with rail temperature extremes of nearly 120 °F (49 °C) in the summer sun.

Joints are used in continuously welded rail when necessary; instead of a joint that passes straight across the rail, producing a loud noise and shock when the wheels pass over it, two sections of rail are sometimes cut at a steep angle and put together with a gap between them - a breather switch (referred to in Britain as an expansion joint). This gives a much smoother transition yet still provides some expansion room.

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