All the victims hailed from Tamil Nadu.
Four sanitary workers, including two women, were killed after they were run over by the Thiruvananthapuram-bound Kerala Express near Shoranur Railway station on Saturday evening.
The incident took place when the workers were clearing garbage from the railway track near the Shoranur bridge - located a few metres from the railway station in the Palakkad district.
The train was going towards Kerala from Delhi, officials said.
According to the police, the victims, who were contracted by the Railways for sanitation duties, were thrown from the track due to the collision's impact, reported news agency PTI.
While three bodies have been retrieved from the incident spot, efforts are underway to locate the body of the fourth victim - who is suspected to have jumped into the Bharathapuzha River, the police said.
All the victims hailed from Tamil Nadu, officials said.
"The workers may not have noticed the approaching train, which resulted in the accident, but further investigations are underway," a Railway Police official told PTI.