The incident at Elsenham, Essex was the perfect example, and now has pedestrian gates that lock when the road barriers are shut.
It was only possible to buy tickets on one side of the track, so northbound passengers had to cross the rails twice. (since the accident a ticket machine has been installed on the northbound side)
The barriers were closed and the sirens went off.
The train arrived and stopped.
Two girls who needed tickets opened the pedestrian gates and crossed.
They were hit by a second train that they hadn't expected. The curvature of the track gave only three seconds visibility of an approaching train.
http://www.bbc.co.uk/news/uk-england-essex-17378565