A molester was jailed for four weeks for groping an MRT passenger after a judge poured scorn on his claims that another man was to blame.

The evidence "strongly suggests that the unknown man is just a figment of the accused's imagination or a red herring in the defence case", said District Judge Terence Tay in decision grounds released on Friday.

Instead, the court found student Yeo Loo Yee, 23, guilty of a series of sustained acts where he touched a woman's left buttock with his hand and an unspecified body part.

The offence took place when Yeo and the victim boarded a train on the Circle Line at Bishan station headed towards Caldecott station at about 8am on July 7, 2014.

The victim felt someone's body touch her left buttock and the contact was too long to be accidental. She saw Yeo, who was standing beside her, through the reflection on the glass of the MRT train doors, and moved a little forward to create more space. She also pushed her handbag backwards to cover more of her back.

Later, however, she felt someone's hand touch her buttock and turned around to see Yeo standing about 20cm behind. She did nothing at that point.

Between Marymount and Caldecott stations, she again felt someone touch her left buttock. This time, she told Yeo: " Hello, have you touched enough?"

The judge noted that Yeo responded with an amused look but did not deny the allegation. The woman later lodged a police report.

In a statement last year to police some eight months after he was arrested, Yeo said the train was very packed at the time and he rubbed the woman because she had squeezed into his space.

There was no mention then of the so-called " unknown man" and the judge found it "unbelievable" that Yeo did not bring him up earlier. He said Yeo came across as an "untruthful and evasive witness" while the victim was an "honest, reliable and cogent witness" whose testimony was " unusually convincing".

On being convicted, his lawyer Jayakumar Naidu urged the court to impose a fine, as Yeo was a student working part-time.

Deputy Public Prosecutor Eunice Ng called for at least six weeks' jail, pointing out that Yeo was placed on 15 months' probation in 2014 and 80 hours' community service on an outrage of modesty charge.

The judge said Yeo's acts were not a "mere slap on the victim's buttock" and it was even more aggravating that the offence was done just four months into his probation. He added that a jail sentence was warranted, noting that with the push towards a "car-lite" society, more people are expected to use public transport, including trains.

"Therefore, it is necessary that passengers feel sufficiently safe when taking the MRT train. They should not have to subject themselves to molestations or other forms of bodily infringements on the pretext of the necessarily crowded conditions on the MRT trains," he added.