It seems that the answer to your question is that the continuously-welded rails work only within a relatively narrow temperature range.
As mentioned at this link, the continuously-welded rails are made to be stress free at a specified temperature, for example 27 degrees Celsius in UK.
At lower temperatures the rails contract, which does not have bad consequences unless the temperature becomes so low that the rails will crack, because their strength has been exceeded by the contraction stress. At very low temperatures the trains may need to reduce their speed, so that the rails will not crack due to the additional stresses caused by a passing train.
At higher temperatures, the rails dilate, and when the temperature is high enough they will buckle. Due to that, the trains may need to reduce the speed, or even stop and wait for the rails to become colder.
At least a part of the rails from Germany have also been updated with continuously-welded rails.
In one of my trips, it was a hot summer day and because of the rail buckling all the trains had huge delays. Even if I should have arrived at the Frankfurt airport about 3 hours before my flight, due to the train delays caused by the hot weather, I have arrived so late that I have lost the flight.
As mentioned at this link, the continuously-welded rails are made to be stress free at a specified temperature, for example 27 degrees Celsius in UK.
At lower temperatures the rails contract, which does not have bad consequences unless the temperature becomes so low that the rails will crack, because their strength has been exceeded by the contraction stress. At very low temperatures the trains may need to reduce their speed, so that the rails will not crack due to the additional stresses caused by a passing train.
At higher temperatures, the rails dilate, and when the temperature is high enough they will buckle. Due to that, the trains may need to reduce the speed, or even stop and wait for the rails to become colder.
At least a part of the rails from Germany have also been updated with continuously-welded rails.
In one of my trips, it was a hot summer day and because of the rail buckling all the trains had huge delays. Even if I should have arrived at the Frankfurt airport about 3 hours before my flight, due to the train delays caused by the hot weather, I have arrived so late that I have lost the flight.