A terrifying gang assault on Sydney train passengers has left two international students seriously injured and caused a media storm in China.

The alleged robbery, including racist taunts, drew a social media pledge from former foreign affairs minister Kevin Rudd and led to emergency talks at Sydney's Chinese consulate general.

Will try to approach police ... a screen grab of Kevin Rudd's message on Weibo.

Police said six people, aged 14 to 18, robbed passengers on a train between Central and Rockdale about 12.30am yesterday.

Officers were called to Rockdale station about 15 minutes later, where they arrested three men, two aged 18 and one 19, a 14-year-old boy and two girls, aged 16 and 17.