Most of the injuries were slight but five people were hospitalized following the accident shortly after midnight Thursday at the Puerta del Sol station, the city's emergency rescue service said.
A train which had just taken on passengers and was preparing to leave was struck from behind by an empty train whose driver had failed to see a red light.
The empty train was moving at less than 20 kph (13 mph) because of a mechanical problem, but still triggered panic in the train it hit. Passengers screamed and tried to claw doors open, newspapers reported.
"It was a tremendous bump. It pushed an entire car into the tunnel," Miguel Jimenez, driver of the train that was hit, told El Pais.
The head of the Russian Finance Ministry, Anton Siluanov, said that the Americans would suffer additional losses if they impose sanctions on Russia's public debt