At least two people were killed and more than 100 injured when a passenger train carrying 800 people collided with a truck, derailing eight carriages, Russian media reported.
Dramatic images show the wagons lying twisted and damaged along the track after the collision.
The train was travelling from Kazan in Tatarstan to Adler on the Black Sea when it derailed in the southern Volgograd region, near the Kotelnikovo station, about 1,200 km (750 miles) south of Moscow, the Emergencies Ministry said.
“Paramedics are on the scene,” the ministry said, adding that 324 emergency workers were involved in the operation. Emergency helicopters have also been called in, it said.
Passengers were trapped in the carriages as those who managed to escape tried to free them before emergency services arrived. Some were trapped under the train.
Passengers were trapped in the carriages as those who could escape tried to free them
Dramatic images show wagons twisted and damaged next to the track after the collision
Shocked bystanders gather around the train as people rush to help those trapped inside
The mangled train car can be seen after the collision
The Mash Telegram channel reported that at least two people were killed, but the death toll could rise. The Interfax news agency said that up to 100 people could be injured.
An unverified video on Telegram showed at least four derailed wagons, some of which were twisted. People climbed out of a wagon that was lying on its side next to the tracks.
Mash published a video showing the battered remains of the cabin of a Kamaz truck next to the derailed train.
TASS cited unidentified sources within Russian law enforcement as saying that 20 people were injured when the train collided with a Kamaz truck.
According to other media, 37 people, including six children, were initially injured in the train crash, four of whom were hospitalized.
Passengers posted shocking images of carriages lying on their sides at the scene.
The train was traveling from Kazan to Adler, part of the Black Sea tourist center of Sochi. The journey was supposed to take 47 hours.
The seriously injured truck driver Aslambek Usmatov, 43, was taken to hospital with serious injuries.
The footage shows him injured and confused after the crash, near the wreckage of his truck’s cab, which is 50 feet (15 meters) away from the rest of his vehicle.
There are 324 specialists from the Ministry of Emergency Situations and 109 pieces of equipment working at the site where the passenger train derailed in Kotelnikovo.
Two Mi-8 emergency helicopters were sent to the crash site.
A criminal investigation has been launched.
“All circumstances of the incident are being established,” said Interior Ministry spokeswoman Irina Volk.
A female witness said: “This is a total nightmare.
“A KAMAZ [truck] flew over [the railway line].
“People are trying to climb out, this is a nightmare.”
A second woman said, “Look, what a nightmare.
“This is our railway crossing in Kotelnikovo.
“Look, there are carriages that have collapsed.
“People are climbing out, what a horror.”
A passenger on the train said: “There was a KAMAZ about 400 metres away as we approached the crossing.
“The crew used the emergency brake at a speed of 95 km/h [60 mph].
“There was a loud bang.
“All we could see was a cloud of dust.
“I saw five carriages – one upside down, one on its side, three at a 45-degree angle.”
He said the engineer and his assistant were still alive.”
Last month, two million people were killed and 70 injured when a tourist train crashed from the harsh Arctic mining town of Vorkuta to the Black Sea resort of Novorossiysk.