Pol Gen Aek Angsananont, the deputy national police chief, was quoted as saying that the SRT employee was a friend and colleague of Wanchai Saengkhao, who confessed to the rape.
The 19-year-old man worked as a porter and had been drinking with Mr Wanchai before the rape, said Pol Maj Gen Thanet Soonthornsook, the Prachuap Khiri Khan police chief. He quoted the man as saying that he had kept a lookout while Mr Wanchai raped the girl in her compartment.
The man was charged with abetting the rape and is being detained at the Prachuap Khiri Khan prison.
The accused was said to have been among four SRT staff who had bee drinking alcoholic beverages on the Surat Thani-Bangkok train before the rape. He was identified as Nathakorn Chamnan.