All of them were given first aid and shifted to a nearby government hospital, they said.
There are no casualties, Southern Railway said, adding all four lines between Chennai Central and Tiruvallur were suspended.
The incident occurred around 8.15 PM, it said, adding "Trivandrum Mail passed signal at danger and hit Chennai Pattabiram siding local EMU train."
Senior railway officials, including Southern Railway General Manager Vashisht Johri rushed to the spot to oversee relief and rescue operations, it said, adding one line has been restored and one train had left Chennai Central.
Railway and police officials had initially ruled out any injuries.