Adver reporter Elly Roberts was on board. She said: “It just felt like the train was changing tracks at first. Then it started to sway a bit and loud crunching could be heard.
“Passengers originally thought the train had gone over a tree.
"Some people were worried and said they had to hold on to the arm rests.
“But it was all over so fast, in less than five seconds.
"I’m just so grateful nothing worse happened.”
The driver told passengers the bridge had collapsed and caused “substantial damage” to the train.
The front carriage engine was 'completely damaged'.
The rear carriage is still intact.
Network Rail said the train had broken down near the Wiltshire station. It was thought the train had hit something on the track, possibly debris from a nearby railway bridge.
Engineers are currently examining the bridge for any signs of damage.
Network Rail said the Dunch Lane bridge had not collapsed.