* Train carriages derailed, four rescued from collapsed building

* Tsunami warnings cancelled, chip foundries unaffected

* More than 600 trapped by blocked mountain roads

* Quake follows 6.4 magnitude tremor on Saturday (Adds details of casualties)

By Ben Blanchard and Yimou Lee

TAIPEI, Sept 18 (Reuters) - A 6.8 magnitude earthquake hit the sparsely populated southeastern part of Taiwan on Sunday, the island's weather bureau said, derailing train carriages, causing a convenience store to collapse and trapping hundreds on mountain roads.

The weather bureau said the epicentre was in Taitung county, and followed a 6.4 magnitude temblor on Saturday evening in the same area, which caused no casualties.

The U.S. Geological Survey measured Sunday's quake at a magnitude 7.2 and at a depth of 10 km (six miles).

Taiwan's fire department said one person had died and 146 were injured by the quake.

All four people were rescued from a building that collapsed in Yuli, while three people whose vehicles fell off a damaged bridge were rescued and taken to hospital.

The Taiwan Railways Administration said six carriages came off the rails at Dongli station in eastern Taiwan after part of the platform canopy collapsed, but the fire department said there were no injuries.

More than 600 people are trapped on the scenic Chike and Liushishi mountain areas by blocked roads, though there were no injuries and rescuers were working to reopen the roads, the department said.

The U.S. Pacific Tsunami Warning Center issued a warning for Taiwan after the tremor but later lifted the alert. Japan's weather agency lifted a tsunami warning for part of Okinawa prefecture.

The quake could be felt across Taiwan, the weather bureau said. Buildings shook briefly in the capital Taipei, and aftershocks have continued to jolt the island.

Science parks in the southern cities of Tainan and Kaohsiung, home to major semiconductor factories, said there was no impact on operations.

Taiwan Semiconductor Manufacturing Co (TSMC) , the world's largest contract chipmaker, said there was "no known significant impact for now".

Taiwan lies near the junction of two tectonic plates and is prone to earthquakes.

More than 100 people were killed in a quake in southern Taiwan in 2016, while a 7.3 magnitude quake killed more than 2,000 people in 1999. (Reporting by Ben Blanchard and Yimou Lee; Additional reporting by Martin Quin Pollard, Sam Nussey and Anirudh Saligrama; Editing by Christian Schmollinger and Stephen Coates)