It is not entirely clear where the 50% claim comes from. One possibility is that the poster was confused by a datapoint on HS2’s website, which states that there are 65 miles (105 km) of tunnels. However, this is counting the total length of the tunnels on both legs of a 280-mile return journey. Sixty five miles of a 140-mile track is indeed close to half (46.4%), however, the more correct way of measuring would be to take 65 miles of the 280-mile track instead (23.2%).
