IG North Zone Syleendra Babu said seven persons were killed in the collision that saw both the trains coming on the same track. The toll may go up.
Additional Division Railway Manager in Chennai Usha Venugopal said seven bodies have been recovered from the three mangled bogies which were thrown off the track following the incident at around 9.40 pm.
Four other bogies of the two trains were also affected. The staionary Arakkonam-Vellore passenger train was waiting for the green signal near Sitheri Station when the Electric Multiple Unit (EMU) running between Chennai Beach and Vellore rammed into it from behind.
A railway spokesman said 85 injured people have been admitted to various government hospitals in Arakkonam and Vellore.
He said the injured have been rushed to local hospitals. Several passengers have been trapped inside two bogies which derailed following the collision, railway police said.
Rescue workers used gas cutters to help pull out the trapped passengers from inside the derailed bogies. Relief trains and two teams of National Disaster Response Force (NDRF) have been rushed to the accident site.
However, rescue and relief efforts were hit by heavy rains and darkness in the area. Train traffic on the busy Chennai-Katpadi section has been disrupted and several trains leaving Chennai Central affected.
Several trains, including Coimbatore-bound Cheran Express, have been cancelled, railway sources said.