The phrase "transition zone" refers to an area or space where one thing changes or transforms into another. It is the zone between two different stages or regions. Full definition
Transition zones require four to eight times more maintenance than ordinary rail tracks. (sciencedaily.com)
The sloping transition zone between them, however, represents a belt of ignorance. (scientificamerican.com)
Much has been written on how to maintain this infrastructure, particularly in the difficult transition zones where trains leave land to ascend bridges over water. (sciencedaily.com)