While the London Underground dates back to 1863, its structural components are divided into "sub-surface" lines and deep-level "Tube" lines. The is officially the youngest traditional deep-level Tube line on the entire network. History and Birth
If you need a niche scientific meaning:
In the context of London’s world-famous rapid transit system, the "youngest tube" refers to the most recently built line.
The London Underground is a frequent target for world records, particularly regarding age and speed. :
Studying the youngest lava tubes allows scientists to understand the primary successional stages of life, as well as the rapid cooling processes of molten rock. A. Microbiology and Colonization