The 20635 / 20636 Ananthapuri Express is an express train running between Kollam Junction and Tambaram via Thiruvananthapuram Central, Tirunelveli Junction, Virudhunagar junction, Madurai Junction, Tiruchchirappalli Junction and Villupuram Junction through the trunk line and chord lines in southern India. It is a daily overnight Superfast Express service.

The inauguration of this train was on 30 June 2002 operating six days a week. Later in 2005 it was made daily train. In the beginning it ran between Chennai Egmore and Thiruvananthapuram Central. It was named Ananthapuri after the city Thiruvananthapuram. In 2017 railway budget, it was extended to Kollam with effect from 1 November 2017.

In total 23 coaches are there. It includes One AC First Class (H),Two AC 2 Tier (A),Three AC 3 Tier (B), Eleven Sleeper Class (SL),Four Unreserved general sitting coach (GS) and Two (SLRD).

The Ananthapuri Superfast Express between Chennai Egmore and Kollam has a rake sharing agreement (RSA) with Uzhavan Express (16865 / 16866) between Chennai and Thanjavur. The primary maintenanced by the four rakes of the express at Tiruchchirappalli (Trichy) coaching depot.

It is very very likely to get lhb coaches by first quarter of 2026. The coach composition will be 10 sleeper coaches, 4 3 ac coaches,3 2ac coaches 1 1 ac coach and 3 Unreserved coaches and 1 car generator(22 coaches).

The engine of 16723 Express caught fire at Kollam Junction on 16 July 2018.