A 15-year-old girl has been arrested over an assault on a 71-year-old woman at a Perth railway station.

Police allege the youth approached the victim and her 65-year-old sister-in-law as they got off a train at Woodbridge station on Thursday, and asked for money.

The women refused and a short time later, the teenager grabbed the 65-year-old woman from behind.

Both women managed to fend her off, but as they walked away, the offender continued to try to grab at the women, then allegedly punched the 71-year-old to the head several times.

The women managed to seek refuge in their vehicle while the offender hit at the passenger side window.

Two unidentified women then appeared and pulled her away.

On Monday, police said investigators had arrested the teenager, from the nearby suburb of Stratton, over the weekend.

The youth will be referred to a Juvenile Justice Team under the provisions of the Young Offenders Act.