many people consider the oldest rapid transit system of any kind to be the metropolitan railway in london, the capital of the united kingdom. the first part of what would eventually become the london underground started building in 1860, and the first part was opened in 1863. the first underground trains were powered by locomotives that burned coal, and the smoke made many people suffer from choking in the tunnels. later trains on the line ran on electric power. this line, now part of the metropolitan line, is still running today. another railway line in london, the city and south london railway, was the first rapid transit line to use trains that run on electricity. this part opened in 1890 and today is part of the northern line. several more underground railways were built in london, and today the underground, which is called the "tube" for its small trains and tunnels, has eleven lines, some of which run on track that is used by national rail trains.