American train attack witness in Paris hospital

French President Francois Hollande bids farewell to US airman Spencer Stone (Michel Euler/AP)

An American, who tackled a so-called Islamic State operative aboard a high-speed train, has gone into hospital, casting doubt on whether he will be able to testify in the trial.

Spencer Stone, then aged 23, was among passengers who helped subdue gunman Ayoub El Khazzani aboard the Amsterdam to Paris train on August 21 2015.