Rescuers digging through the rubble in the collapsed train tunnel in Nakhon Ratchasima were about one metre way from one of the three trapped workers, the provincial governor said on Wednesday morning.

Soil and rocks continued to fall in the under-construction tunnel and the safety of both rescuers and victims was of concern. The rescue tunnel had to be shored up as they progressed. 

Falling soil had opened overhead holes and sunlight reached rescuers on Wednesday morning, the governor said. 

Officials and rescuers remained optimistic as every scan detected the vital signs of all three trapped men, he said.

A scan showed that rescuers were a bit over one metre away from one victim. Another was about four metres away and the third eight metres, he said.

Two of the trapped workers are Chinese, and the other is from Myanmar.

They have been trapped since the tunnel collapsed on Saturday night. The tunnel is part of the Sino-Thai high-speed railway being built between Bangkok and Nong Khai province.