One of the victims had a hand cut off and another had their skull split open, the source added, asking not to be named.
Two others were more lightly injured.
Several people, some of them minors, were involved in the fight that broke out around around 8 am (0700 GMT), the source added.
The incident happened while the RER suburban express train was at the station of Ozoir-la-Ferriere southeast of Paris.
Since the train was halted at a station, the violence did not affect traffic on the RER E line, which runs east-west through Paris and its suburbs, state rail operator SNCF said.
