9NEWS has obtained security video of gangs of thieves targeting vulnerable Sydney commuters.

The gangs are seen pouncing at train stations, violently snatching mobile phones and cash.

In one CCTV video, a woman is seen falling down an escalator, with her attackers stomping over her.

In another video, thieves at Blacktown station steal a man's mobile phone and calmly walk away after repeatedly punching him.

Police admit the footage is confronting to watch, but they claim our public transport system is getting safer.

"If you're committing those sort of crimes, then you're going to get caught," Supt. Jason Joyce said.

Police point to the falling number of robberies on our trains.

Since 2013, this number has dropped almost 30 percent.

Police say this is due to the increased police presence on our rail network.

The amount of service officers has doubled to almost 600 officers out on the beat for the NSW Police Transport Command.