Pol Colonel Krissana Patanacharoen, deputy spokesman of the Royal Thai Police, said the national police chief, Pol General Chakthip Chaijinda, ordered police to work faster in gathering evidence to seek an arrest warrant.
Chakthip wanted the suspect to be arrested soon to create confidence for Bangkok commuters, the spokesman added.
A woman passenger, whose name was withheld, filed a compliant at the police station at 4.40pm, saying she was attacked by a young man.
She said she was walking on the stairway to the M floor of the Hua Mark railway station at 7.40am and a man, who appeared to be about 20, used his arm to lock her neck from behind and grabbed her breast. She managed to break free and call for help, and the man ran away.