Members of a gang responsible for a train graffiti attack at Otford allegedly threatened to kill rail staff if they intervened.

At 12.30am on Saturday, a group of five people stopped a Wollongong-bound train at Otford station by placing a piece of timber and a milk crate on the tracks.

They then spray-painted the side of a carriage while the driver and passengers remained inside, before running off.

Police believe they escaped in a car that had been parked close to the train station.

Several people have contacted the Mercury claiming the vandals were armed with more than just cans of spray paint.

One said the gang members were carrying baseball bats, which were used to threaten to bash or kill the driver and passengers if they made any attempt to move the train.

However, a police spokesman said the incident report made no mention of the gang wielding any weapons.

Rail, Tram and Bus Union NSW secretary Alex Claassens said the Otford incident was the latest in a series that highlight failing security measures on the state's rail service.

"In the last few weeks alone we have seen at least three incidents, which have left one train driver with a broken nose, a number of guards with facial injuries, another guard injured by a thrown fire extinguisher and other workers threatened and bullied while vandals held up their train," Mr Claassens said.

"These horrifying attacks on transport workers must be stopped."

Mr Claassens blamed the government's decision to replace transit officers with the Police Transport Command (PTC) as part of the reason for the decline in security.

"The police have struggled to fill their complement and as a result, we have a significant deficit of security on the network," Mr Claassens said.

"The 600 transit officers who worked exclusively on the trains have been replaced by only around 400 police officers who work across trains, buses and ferries."

A Transport for NSW spokesman said the PTC had made 4220 arrests, laid 7935 charges and issued 81,913 infringements since its creation in 2012.

It had also made more than 600 arrests for graffiti offences on public transport in that time.

Meanwhile, Transport for NSW said cleaning up graffiti such as the spray-painted carriage at Otford cost the state millions of dollars.

"Graffiti on trains and within the rail corridor is an ongoing challenge," a spokesman said.

"Last year, cleaning up graffiti on the rail network cost taxpayers around $30 million."