The worst on record is very recent: when the big Southeast Asian Tsunami hit a train on Sri Lanka, 1800 people may have perished.

Before that, the three worst I heard of: a train falling onto a river in India (c. 800 dead), a train in the eighties in the Soviet Union rushing into a cloud of gas spewing forth from broken pipelines and bringing it to explosion (c. 600 dead); and a French military holiday train during WWI, passing the Italian-French border, on which the military officer threatened the engineer with court martial if he doesn't proceed downhill with an overwheight train, on which first the hot and failing brakes caused fire, then all fell into a ravine (c. 5-800 dead).

_{*Lunatic*, n.
One whose delusions are out of fashion.}