How about:
Hither Green 1967 (broken rail following track relaying)
Armagh 1889 (overloaded train, no automatic brake)
There haven't been many accidents ever on the main line caused by broken sleepers, as the number of consecutive failures required precludes the gauge getting bad enough.
Overloaded freight trains running out of control is generally the stuff of Holywood, though this one technically counts (though it wasn't overloaded).
Just an incidental detail regarding the Hither Green derailment of '67.... the late Robin Gibb was a passenger on this train and had only been in the country a few weeks at the time.
One particular freight derailment which comes to mind involved a Saltley driver known as 'Stacker Steadman'... I don't have a date or any real detail on this one but he went down the Lickey a tad too quick, was put in the Goods Loop at Bromsgrove and still couldn't stop, ending up going through the traps at the south end... resulting in several of his wagons being stacked up on top of each other. One of our (ex-SY) drivers knows more about it so I'll ask next time I see him.