The app, which was launched on Wednesday, will give commuters a good idea of how many people are in each carriage as the train pulls into the station.

The app only works with the new Waratah trains that operate on Sydney’s T1, T2, T3, T5 and T8 lines.

This is due to the technology on those trains, which is not a feature of the Oscar cars on the South Coast line.

That technology on the Waratahs has to do with the ability to weigh each carriage, which is what the new app relies on.

The app works out how many people are in each carriage based on the weight of each carriage as measured by airbags that sit under each axle of the train.

As the doors close at each station, the weight is recorded and sent to the app via the 3G network.

READ MORE: Steady stream of complaints about train crowds

The app then uses an algorithm to calculate how many people are in each carriage, based on the average weight of a NSW train passenger.

Users then see a set of boxes representing the number of carriages on the train.

Boxes coloured green indicate that seats are available, orange boxes mean it’s standing room only in that carriage while red means it’s a tight squeeze.

NSW Premier Gladys Berejiklian said the service brought trains into line with buses.

“Since 2016, customers have had access to bus occupancy data to help choose the right service for them,” Ms Berejiklian said.

“Now train customers can see how full their train is before it arrives.”

Transport Minister Andrew Constance believed it would reduce crowding on carriages by spacing out commuters.

“Letting customers know which services might be full or which carriages still have seats will make commuting choices easier and help ease congestion,” Mr Constance said.