The four women, in their early twenties, were traveling between Marseilles and Paris when they were attacked at 11am.
A police spokeswoman told USA Today that the suspect did not yell any out any terror-linked threats.
The spokeswoman spoke on condition of anonymity, per the custom of the French judicial system.
14 firefighters rushed to the scene in four emergency vehicles as the horror unfolded. She said a 41-year-old female suspect has been arrested.
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"He's been very caring and it's meant a lot to her to have him by her side". "He is happy because she is healthy". She gave me the ultimate gift and sacrifice by donating her kidney to me.
No other information on where the US tourists were from was released.
Marseille is a port city in southern France that is nearer to Barcelona than Paris.
French reports say the incident is not being treated as terror-related and witnesses said no slogans were shouted during the attack.
In previous incidents in Marseille, a driver deliberately rammed into two bus stops last month, killing a woman, but officials said it wasn't terror-related. He told police he acted for the sake of the Islamic State gathering.