offshoot

Something which shoots off or separates from a main stem or branch of a plant.

the offshoots of a tree

Something which develops from something else.

an offshoot of a criminal organization

The reason is that the Berks & Hants Railway Company, under the powers of which the line was built, also included the Reading-Basingstoke section. The company was an offshoot of the G.W.R., which absorbed it before any section was opened.