The land owned by our railways, bordering 20,000 miles of track, is home to hundreds of different plants and animals, including many rare and endangered species.
Enter the railway industry's first tree specialist, Neil Strong.
Neil's task is to manage the enormous area of land owned by Network Rail.
His estate runs through all of the country's national parks, takes in every different habitat found in Britain, and contains 330 designated sites of special scientific interest.
And lots and lots of trees.
Mark Stephen explores the wealth of plants and animals that live along Britain's railway lines and discovers just what happens when the wrong kind of leaves get on the track.