A bus commuter became suspicious when cleaner Lim Wai Sin, 53, chose to sit next to a 12-year-old school girl on July 31 this year despite there being numerous empty seats.
When the bus neared the second stop after the Punggol interchange, she started fidgeting and Mr Mohd Nordin Abdul Muthalif, 38, who was keeping a close eye on the pair, went forward to ask if she was alright.
She said she was being molested. On Wednesday, a district court sentenced Lim to 10 months in jail. He had pleaded guilty to using criminal force to outrage the modesty of the primary school student.
Mr Mohd Nordin, a crane operator, told police that he saw the cleaner rubbing his elbow against the girl's rib cage on SBS Transit service 85 at 6.30am that day. He had then asked the bus driver to call for the police and Lim was arrested.
Assistant Public Prosecutor Puvaneswari Sandirasekaran told the court that Lim had been jailed for six months in 2010 for committing three similar offences.
He had also been jailed between two weeks and three months for insulting the modesty of women in 1993, 2001 and 2006, she added
District Judge Shaiffudin Saruwan said that in determining the sentence for Lim's latest offence, he had considered that the cleaner suffered from frotteurism.
This refers to a paraphilic interest in rubbing some part of one's body against young persons for sexual gratification.
The judge also noted that Lim, who has an IQ of 58, had "mild mental retardation" and that he had rubbed the victim's rib cage and not the chest or breast areas, which would entail heavier penalties.