Rail transport in England is the oldest in the world: passenger railways originated in England in 1825. Much of Britain's of rail network lies in England, covering the country fairly extensively, although a high proportion of railway lines were closed in the second half of the 20th century. There are plans to reopen lines such as the Varsity Line between Oxford and Cambridge. These lines are mostly standard gauge (single, double or quadruple track) though there are also a few narrow gauge lines. There is rail transport access to France and Belgium through an undersea rail link, the Channel Tunnel, which was completed in 1994. Crossrail, under construction in London, is Europe's largest construction project with a £15 billion projected cost. High Speed 2, a new high-speed north–south railway line, projected in 2015 to cost £56 billion, but estimated in 2020 to be almost double that figure, is to start being built in 2020.