The 28-year-old man from Zimbabwe has not been named but was remanded in custody following an altercation on the 10.04pm train to Reading on June 18.
He was detained on suspicion of racially aggravated assault, sexual assault and obstructing police.
British Transport Police officers are now appealing for information from passengers on the train who may have witnessed the suspected incident.
They particularly want to speak to a woman who they believe may have information about another offence.
PC John Hughes said: “We are keen to hear from a woman who was on the train, who we believe may have important information about a separate offence.”
“We would also like to hear from any other witnesses who have not yet spoken to police.”