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.
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