That would be Moscow in Russia with 2.392 billion annual riders. Second is Paris, France with 1.388 billion and third London, England with 1.09 billion.
Moscow.
Busiest
which city has underground railway system streatching 265kmand 170 station
Kolkata
Subway.
Both as a bomb shelter and a nuclear radiation fallout shelter. OR... The Busiest.
London, UK in 1863
The Paris Métro is an electric underground railway system; the term metro is also used in other countries to define a rapid transport system, not necessarily underground.
The system was labeled "The Underground Railroad", but obviously was neither a literal railway, nor technically underground.
Madhya Pradesh
Madhya Pradesh
the busiest.