The driver on the Red Line train engaged the emergency brakes:
http://www.washingtonpost.com/wp-dyn...062300653.html
The NTSB said there was evidence on the tracks that the brakes had indeed been used, so it looks like it was just too late. Given the issues I mentioned before that would have obstructed the driver's vision (curve, bridge, shadows, chain link fence), it's not really surprising.
A lot of Metro's dirty laundry is also coming out for everybody to see. The NTSB has been on Metro's case a lot over safety issues, but Metro doesn't have a dedicated funding source, so they don't have the money to implement changes.