yobs threw a rock and a traffic cone onto railway tracks from a bridge in Belper.
In separate incidents, both items were dropped from the bridge on Main Street, under which express trains moving at speeds of up to 100mph travel.
Now, Network Rail is warning people about the dangers of throwing things onto railway lines and is asking the town council to spread the word about safety.
As well as the obvious dangers posed to the train, its driver, and passengers, Network Rail pointed out that to clear the track of debris would result in having to stop trains and therefore cause delays.
A spokeswoman said: “Heightened awareness of the problem is perhaps a good place to start.”