A possible major collision was also averted as the Chennai-Trivandrum train coming towards Ernakulam was stopped around 300 metres away from the accident spot following a timely alert, railway official said.
The derailment occurred at 2.55 AM shortly after the 16347 Thiruvananthapuram-Mangaluru Malabar Express left Aluva station and was running at a slow speed.
There was no casualty and all passengers were taken to Thrissur for their onward journey, Railways said.
One woman passenger complained of shoulder pain and was taken to a nearby hospital and after first aid left for home, he said.
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According to railway officials in Thiruvananthapuram, the derailed coaches have not detached or toppled, which is one of the reasons for passengers escaping unhurt.
A Railway official said major collision was averted as the Chennai-Trivandrum train coming towards Ernakulam was stopped on time.
"We alerted the incoming train and it was stopped about 300 metres away. This was an achievement of the railways safety measures," the official said.
Rail traffic towards Ernakulam was expected to be restored by 6 PM today and on the Thrissur route by tomorrow morning, Mishra said.
Railway officials from the state capital and elsewhere are overseeing the relief and rescue operations.
On the mishap, Mishra said it could be due to defect in track or in the coaches.
Chief Minister Pinarayi Vijayan said Railways have arranged special trains and buses for transporting stranded passengers, adding it was suspected that there was some defect in the track.
It was raining heavily when the incident occurred, he said.
A railways release announced helpline numbers: 9746769960, 9746761072, 0471-2320012.
Six long distance trains, including Kanyakumari-Mumbai CST Express, Thiruvananthapuram-Gorakhpur Rapthisagar Express, Kanyakumari-Begaluru Express and Thiruvananthapuram-New Delhi Kerala Express have been diverted via Tirunelveli, it said.
Kerala State Road Transport Corporation (KSRTC) will be providing buses between Thrissur and Ernakulam depending on the arrival and departure timings of trains, it said.