The A75 Gretna to Stranraer road has been closed near Dunragit following a two-vehicle accident.
Dumfries and Galloway Police reported the incident at the Challoch Bridge at about 0930 BST.
Network Rail inspectors are to examine the bridge for any structural damage as a result of the accident.
A police spokesman confirmed that diversions had been put in place and the road was expected to remain closed until the early afternoon.
The bridge has been struck at least three times in the past year during accidents.