Rail service that runs underground. The first underground line in the world was in London, opened in 1863; it was essentially a roofed-in trench. The London Underground is still the longest underground system, with over 400 km/250 mi of routes. Many large cities throughout the world have similar systems, and Moscow's underground, the Metro, handles up to 6.5 million passengers a day.
The number of stripes changed frequently until around 1800. Red, white, and blue became the colours of liberty and an inspiration for other revolutionary flags around the world. Effective date: 19 February 1937.