The train driver spotted the herd of about 100 elephants and used the emergency brakes but the train still hit some of the animals, Indian Railways spokesman Kapinjal Kishore Sharma told the Associated Press.
Five train coaches and the engine derailed following the impact but there were no human casualties, Sharma said.
Veterinarians carried out autopsies on the dead elephants, which were to be buried later in the day.
The accident site is a forested area about 125km southeast of Assam's capital city of Guwahati.
Himanta Biswa Sarma, chief minister of the state of Assam, wrote on X on Saturday that the news of the death of three adult animals and four calves "deeply saddened" him.
Railway tracks in the state are frequented by elephants but Indian Railways said in a statement the accident location was not a designated elephant corridor.
The Rajdhani Express train, travelling from Sairang in Mizoram state bordering Myanmar, was bound for the capital New Delhi with 650 passengers onboard when it hit the elephants.
"We delinked the coaches which were not derailed, and the train resumed its journey for New Delhi. Around 200 passengers who were in the five derailed coaches have been moved to Guwahati in a different train," Sharma said.
Speeding trains hitting wild elephants is not rare in Assam, which is home to an estimated 7000 wild Asiatic elephants, one of the highest concentrations of the pachyderm in India.
Since 2020, at least a dozen elephants have been killed by speeding trains across the state.
In August, India's Ministry of Environment informed parliament that at least 79 elephants had died as a result of such accidents in the past five years, NDTV reported.
Wild elephants often stray into human habitations this time of year, when rice fields are ready for harvesting.
with DPA
