A baby elephant was among the animals killed in the pre-dawn collision in India's Jalpaiguri district some 620 kilometers (385 miles) north of the state capital Kolkata, said V.K. Sood, chief conservator of forests in West Bengal state.
Another elephant is still in a critical condition after the train plowed into part of a herd crossing the tracks, he said.
Concern over similar incidents had prompted railway authorities to restrict train speeds in the area to 25km an hour. However, the railway department has said a speed of 50km an hour was permissible in the area where the accident occurred.
Sood said an inquiry is now underway to determine the speed of the train that killed the elephants Thursday, he said, adding that investigators were also looking at the levels of visibility on the track at the time.
As many as 36 elephants have been killed by trains in the area since 2004, Sood said.
India's former rail minister Pawan Kumar Bansal said in a Railway Budget speech in February the department needed to make special provision to protect what he called "these gentle giants."
"The railway family is deeply grieved by some incidents of death of elephants on railway tracks passing through forest areas. Several measures have been initiated in consultation with the Ministry of Environment and Forests, which I am confident will substantially reduce such accidents and safeguard the lives of these gentle giants," he said.
He said the department had already proposed a bypass rail link around one reserve with a sizable elephant population.
Conservationists estimate that India has a wild population of some 25,000 Asiatic elephants but their habitat is increasingly under pressure as human settlements encroach on nature reserves.
India has recently witnessed a number of cases of wild animals entering urban environments. In one high-profile incident a leopard killed one person and injured two other after wandering into a residential area in northeast India.