The two passenger trains crashed head-on Saturday night near the village of Szczekociny, leaving at least 30 people with serious injuries and one American among the dead.
The U.S. consulate in Krakow told The New York Times the family of the American woman was being notified and that her name was being withheld for the time being.
Polish officials said they wanted to know how the two trains wound up on the same track. One train was headed to Warsaw from Przemysl and the other had left the capital for Krakow.
"We can talk about three potential causes: technical error, human error or an error in procedure," said Piotr Kazimierowski, president of the nation's railroad union.
The collision occurred as Poland prepares to host the Euro 2012 soccer tournament. Thousands of fans from across Europe are set to travel by train around Poland to cheer on their teams, which The Times said would put pressure on the government to find and correct the cause of this weekend's disaster.