Jinda Phntharak, 52, was arrested at her house behind the Nakhon Si Thammarat railway station.
Jinda admitted that she had sold two methamphetamine pills to Wanchai Saengkhao, a bed attendant before the train left for Bangkok.
Wanchai had admitted that he took the drug and raped and killed a teenage girl at her sleeping compartment early Sunday.
The Nation