The Hawk team go up every day during the summer holidays, on the lookout for people up to no good.

Danger

This includes anyone trespassing on railway lines - playing chicken, throwing bricks and missiles at moving trains and blocking train tracks with heavy objects, which can cause them to be derailed.

And kids blading and skateboarding at stations should also watch out - both are banned on platforms and the team looks out for them too.

The air crew film people who look like they're breaking the law with a special camera strapped to the bottom of the aircraft.

Life saving

They radio through to police on the ground who track the offenders and arrest them for causing danger to themselves and others.

Cardiff is one of the worst areas in the country for railway vandalism and although the Hawk isn't cheap - it costs £3,000 a day to run - it saves lives.

Over the last three years 20 children have been killed on railway lines across in the UK.