The lorry driver was trapped in the wreckage of his vehicle after the incident involving the 1505 South West Trains Guildford to London Waterloo service near Oxshott.
Surrey Fire and Rescue service said two passengers had suffered injuries which were preventing paramedics taking them to hospital.
A spokeswoman said: “Two people are medically trapped, meaning they need to be stabilised before they can be taken off.”
British Transport Police said the lorry driver and one passenger were “more seriously injured”.
She added: “The remainder of the passengers remain onboard the train at this time while emergency services attend to the injured.”
Four other people among the 40 passengers suffered minor injuries.
The bridge from which the lorry fell was assessed as safe in the nationwide checks made after the Selby rail disaster nearly 10 years ago.
The checks were ordered after a Land Rover came off the road and on to the East Coast Main Line in February 2001.
Ten people died in that tragedy and more than 80 were injured.
The concrete mixer lorry crashed through a brick wall on a bridge above the busy commuter line, landing on the roof of the third carriage from the back of the eight-carriage train near Oxshott station.
One passenger described on Twitter how he narrowly escaped injury, tweeting: “See this sh*t on TV but it’s actually happening to me.” He added: “We was gonna sit on that carriage too.”