The young man who was injured on Saturday, said to be in his 20s, was still alive when emergency services took him away from the track at Villejuif on the outskirts of Paris.
But his life was in danger, according to hospital services.
Investigators were to look into the exact circumstances of the accident, which took place at 7.30am on Saturday.
Four days earlier a 16-year-old died after being knocked off the roof of a metro train on the overhead tracks near the Eiffel Tower, a favourite spot for trainsurfers, who climb onto trains’ roofs and leap from carriage to carriage.
Videos of the practice are posted online and shared on social media.
In a statement, the RATP transport authority warned that it is “extremely dangerous” and can “lead to death”.
There have been two other deaths in similar circumstances in recent years.
One person was electrocuted in 2015 and a young British man died after suffering injuries to the head while trying to climb onto a carriage on the same line as the teenager who died this week.