The suspect, whose name was withheld due to his age, was apprehended on Wednesday night at his rented room in Soi Pattanakarn 20 in Suan Luang district.
After confessing to the attack, the young man was initially charged with forcibly conducting a lewd act on another person aged over 15 in public.
National police chief, Pol General Chakthip Chaijinda, had earlier this week ordered police to work faster in gathering evidence to seek an arrest warrant, in order to restore confidence among Bangkok commuters.
A woman passenger, whose name was withheld, filed a complaint at the police station at 4.40pm on Monday, saying a young man had attacked her.
She said she was walking on the stairway to the M floor of the Hua Mark railway station at 7.40am when the assailant put her in a headlock from behind and grabbed her breast. She managed to break free and call for help and the man ran away, she said.