The explosion was triggered about 1pm near Koh Sabah Station as a train from Sungai Kolok to Nakhon Si Thammarat was passing, said Pol Lt-Colonel Uab Maisuk, deputy district police chief.

The blast caused no casualties and only damaged a water tank on the train.

Train services in Songkhla province were halted for two hours to allow officials to inspect the damage, according to Thanongsak Phongprasert, chief of the Southern Centre of the State Railway of Thailand.

The railway authority asked police to beef up security at Hat Yai station, the main rail junction in the southern region, he added.

Police suspect militants detonated the bomb to cause trouble in the predominantly Muslim area.

Separatist violence has flared in the South since the beginning of 2004, prompting a harsh response from the government. More than 1,300 people have been killed in the region since then.

In another incident yesterday a merchant was shot dead in Songkhla's Sabayoi district while waiting in his pickup truck near a market for his wife.

Wadeng Salae, 68, was shot three times in the neck and chest. He died instantly, police said.

Eyewitnesses said the gunman simply walked up to the vehicle and fired on Wadeng before escaping on the back of a waiting motorcycle.

Police believe the gunman was a militant intent on maintaining the trouble in the

region.