Which country has the oldest rail work in the world?

The United Kingdom has the oldest national railway system and network in the world, tracing its commercial roots back to early wooden wagonways like the Tanfield Railway from 1725.

https://en.wikipedia.org/wiki/History_of_rail_transport_in_Great_Britain_1830%E2%80%931922

Constructed in 1725 as a wooden wagonway, it is the oldest operational railway in the world. It includes the Causey Arch, which is the world's oldest purpose-built railway bridge. Today, it still runs as a heritage steam line. Established in 1758, it holds the record as the oldest continuously working railway. In 1812, it became the first commercial railway to successfully transition from horsepower to steam locomotives. Opened in 1825 by George Stephenson, this was the world's first public railway to use steam locomotives to transport both cargo and passengers. Opening in 1830, it was the world's first fully twin-track, inter-city passenger railway completely reliant on timetabled steam locomotives. Its terminus, Liverpool Road Station, remains the oldest surviving rail terminal.