The driver of the freight train that hit a passenger bus and other vehicles on a congested Bangkok level crossing on Saturday tested positive for drug abuse and did not have a rail transport licence, according to the rail transport chief.

Pichet Kunadhamraks, director-general of the Department of Rail Transport, posted the allegation online on Sunday night. He said police reported that Sayomporn Sornkul, driver of the train, had a urine test that returned positive for drug abuse.

Mr Pichet also said Mr Sayomporn did not have a train operator's licence from the Department of Rail Transport.

Mr Pichet has ordered alcohol and drug tests for all department employees involved in train operations. He also demanded an investigation into the railway supervisors who allowed Mr Sayomporn, 46, to drive the train involved in the deadly collision on Asok-Din Daeng Road in Bangkok on Saturday afternoon.

Mr Sayomporn has been suspended and faces punishment for a serious disciplinary offence, in addition to legal charges. 

Police said the train driver was initially charged with reckless driving causing death and severe injury. The driver of the bus that stopped on the level crossing faces the same charge.

The collision killed eight people and injured about 30 others.