Thai police said on March 31 that they had charged 70 migrants from Myanmar and six Rohingya Muslims with illegal entry after they were arrested on a train bound for a southern province bordering Malaysia.
The migrants were taken off the train from Bangkok at around 4:30 am in Nakhon Si Thammarat province when they failed to present valid travel documents, said local railway police sub-inspector Mr Kraisorn Boonlum.
"All of the 76 were charged with illegal entry. The Myanmar ones will be deported back to their country, the six Rohingya must first have their nationalities identified," he told AFP.
Mr Kraisorn said the migrants claimed not to know one another and to have boarded the train bound for Narathiwat province at different stations along the route.
"The Myanmar ones said they wanted to find jobs in the south of Thailand but the Rohingya said they wanted to enter Malaysia," he said.
Thousands of Rohingya - a Muslim minority group not recognised as citizens in Myanmar - have fled deadly communal unrest in the country's western state of Rakhine since 2012. Most have headed for mainly Muslim Malaysia.
The six Rohingya migrants, all men in their 20s, and the Myanmar nationals -- mostly adults and a handful of children -- are being held at an immigration centre in Meuang district in Nakhon Si Thammarat.
The authorities are now investigating whether the Rohingya are victims of trafficking.
"The Rohingya said, via a translator, that their relatives in Malaysia paid around 70,000 baht [K2.15 million] each to Myanmar agents (for their passage) -- but the Myanmar ones said they did not pay anyone," Mr Kraisorn added.
Thailand has long been a hub for people-trafficking, with thousands of Rohingya believed to have passed through the kingdom in recent years.
But they normally arrive in the southwest of the country on rickety boats, rather than taking a train.
Myanmar views its population of roughly 800,000 Rohingya - described by the United Nations as one of the world's most persecuted minorities - as illegal Bangladeshi immigrants and denies them citizenship.
Thailand's military junta, which took over in a coup last May, has vowed to crack down on human trafficking. In January it said more than a dozen Thai government officials were being prosecuted for the trade.