(rail transport) A junction where the diverging line descends to a level where it can pass under the main line, the purpose being the avoidance of conflicting movements.
1962 December, “Beyond the Channel: Switzerland: Transformation at Zurich”, in Modern Railways, page 417:
The latter and the Basle-Gotthard freight traffic will both pass through Othmarsingen, but an additional line here and a burrowing junction will keep the Gotthard freights clear.