A breather switch on a TGV
A breather switch, expansion joint or adjustment switch is an intentional gap in railway tracks to allow for expansion in long sections of otherwise unbroken rail. They are placed between very long sections of continuous welded rail (CWR) or at the transition between CWR and jointed track, and commonly in the vicinity of bridges and aqueducts where the infrastructure and track may expand at different rates.
The mating sections are tapered diagonally (unlike a butt joint between two lengths of rail), allowing smoother transitions and higher speeds across the gap.
Breather switches have two advantages over jointed track: they eliminate the small gap between the rails, eliminating or significantly reducing the noise and vibration caused by passing trains, and, where CWR is particularly prone to expansion, they act as relief points to ensure that track does not warp out of shape or cause a sun kink.