A British Transport Police spokesman said a total of 36 people were on the train. Six people are believed to have been injured, one seriously.
The lorry left the road at Warren Lane, a short distance from Oxshott railway station, crashing through a brick wall on a bridge above the busy commuter line.
It struck the third carriage from the back of the eight carriage train, punching a large hole in the metal roof.
Emergency services are at the scene, including six ambulances from the South East Coast Ambulance Service.
Passenger Ade Joshua told BBC News: "In the carriage right next to us where the incident happened, there was a guy trapped with what looked like a broken back, so we were trying to console him as much as we could until the emergency services arrived.
"But we couldn't actually get through to the carriage because the door was blocked as that bit of the train collapsed."
Eyewitness Nikki Karim works in a taxi office near to the crash site.
She said: "There's a lot of police at the station, there's a few minor casualties that are sat at the platform."
She added: "You don't expect this sort of thing to happen."
The incident took place on a branch line, and is unlikely to result in major disruption.
