1. The train leaves the North Philadelphia station and accelerates at maximum power to a planned 80 mph.
2. A rock strikes the train, damaging the windshield.
3. At some point, the engineer is supposed to take the throttle to idle and then apply the brakes to reduce the speed to 50 mph for the curve.
4. However, the engineer is distracted by the rock striking the windshield and doesn't start braking at the correct time.
5. Still accelerating at maximum power, the train continues past 80 mph to 106 mph.
6. The engineer regains concentration, realizes the train is going too fast, and applies the emergency brake.
7. But it's too late. The train tries to go around the curve at 104 mph, derails and crashes.
A Septa and an Acela train were also hit by rocks about the same time. Note that the engineer in the Septa train that was hit was shaken up enough by the incident that he couldn't continue driving the train.
The rock throwing has continued for many years. It's one of those things that the authorities don't worry about until someone dies. With this accident, eight people have died. The problem will now be fixed.