A boat crashed into Kingsferry Bridge shortly after a yachtsman on-board reported seeing a bomb in the water.
The crossing, which links Sheppey with the mainland, was shut while police and inspectors monitored the situation.
The boat hit the bridge after 11am - causing train services running between Sheerness and Sittingbourne to be cancelled.
They were subject to delays and cancellations until around 6.30pm when normal service resumed.
A police spokesman said: "Police were called at about 11am after a yachtsman reported seeing an object in the river that may have been a mortar shell.
"Officers attended the scene where the item could not be located."
That same yacht is then thought to have hit the bridge and run aground.
It then managed to free itself and continued on its way.
Replacement bus timetables were drawn up to cater for passengers travelling between Sheerness and Sittingbourne.
Rail services were also disrupted in Medway where police were talking to a man stood on a bridge above the line at Gillingham.
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