To be pedantic, there were in 2019 9824 miles of route and 19,291 miles of track; 8106 miles of track (3339 miles of route)were electrified. The distinction should be carefully observed, and certainly this forum ought to use the terms in their precise meaning. The percentage of track mileage electrified (42%) is higher than the percentage of route mileage, because the electric lines include more multiple track main lines (2.42 miles of electric track per mile of electric route); 11,185 miles of track remain unelectrified.
See a very good article in the latest Modern Railways (April issue), by Richard Harper, calculating how many miles a year we would have to electrify to decarbonise the railways by 2050. He has a fascinating statistic: since the Mersey Railway was the first steam line to electrify its system in 1903, we have reached 8106 miles of electrified track in 115 years, or 70 miles of track a year. He doesn't include the mileage of former electric track closed (e.g. Woodhead), de-electrified (e.g. the NE Shildon line) or converted to metro operation (e.g Tyne and Wear or Manchester Metrolink), as not part of National Rail. He doesn't include the London tubes and sub-surface lines either, I assume.
