What is the principle of rail gap reservation ?

What is the principle of rail gap reservation ?
Ensure that the rail gap is not tightened when the rail temperature rises, and the bolts are not under force when the rail temperature drops
What conditions should be satisfied for the rail gap reservation?
To meet the needs of expand with heat and contract with cold of rails, gaps should be reserved at the rail joints. The reserved rail gaps should meet the following conditions:
1)When the rail temperature reaches the highest local rail temperature, the rail gap should be greater than or equal to zero to prevent the rail end from being squeezed, in order to prevent the expansion of the track track due to excessive temperature pressure;

2)When the rail temperature reaches the local minimum rail temperature, the rail gap should be less than or equal to the construction rail gap, so that the joint bolts are not subjected to shear force, to prevent the joint bolts from bending or breaking.