It is Sunday morning - hardly peak time - yet it is already standing room only as the train heads towards Parramatta and the Blue Mountains.

Ms Fearnley regularly travels on the train on Sunday and said it was "next to impossible" to find a seat.

"We're jammed in like sardines or I have to stand up," she said. "I mean, we're heading into the western suburbs of Sydney, not into the Bronx."

Tourism operators in the Blue Mountains have also expressed concern about overcrowded trains.

"If something is not done about it, sooner rather than later, it will potentially have a negative effect on tourism because the word will spread," said Jason Cronshaw, the managing director of Fantastic Aussie Tours, which operates the Blue Mountains Explorer Bus.

Besides the packed carriages, Ms Fearnley also said the toilets were "absolutely disgraceful".

"I have actually gotten off the train to go to the bathroom and caught another train," she said. "But then you lose time."

Ms Fearnley said she would not let her son use the toilets: "I've changed him standing up on the train but that is a whole different problem. You never know who's looking."

Some passengers on the Blue Mountains line will stand for an hour or more - jostling for space in the train's narrow aisles and stairs with luggage, prams and bicycles - to reach popular tourist destinations such as Leura and Katoomba.

The situation is even worse in the afternoon as passengers fill four-carriage trains that sometimes run only once an hour back to Sydney's Central Station.

Most passengers seem resigned to the packed carriages, but some tourists express displeasure at having to stand.

"It's quite sad when you think about it," Ms Fearnley said. "I mean we should be showing some pride in what we have."

Overcrowding on weekend train services to western Sydney and the Blue Mountains appears to be a growing problem.

"One older lady, forced to stand up all the way to the Mountains, collapsed in the vestibule," Christopher Webber wrote on the Facebook Cityfail group in May.

"Forcing people to stand for long journeys presents obvious safety issues. It doesn't need to happen, and it shouldn't happen," said Bob Nanva, the national secretary of the Rail, Tram and Bus Union.

Mr Nanva, who lives in Faulconbridge, said he saw overcrowding on the Blue Mountains line every weekend.

"When we're trying to sell the Blue Mountains as a global tourist destination, this sort of service is not just disappointing, it's embarrassing," he said. "We have people travelling to a world heritage tourism asset on a third world rail service."

The Labor MP for Blue Mountains, Trish Doyle, said weekend trains to the Blue Mountains were "chronically overcrowded", with passengers standing for many hours or sitting in the aisles.

"Every weekend, train passengers are crammed in like sardines on four carriage trains on the Blue Mountains line," she said on Facebook. "Matters were made even worse during the recent school holidays and Easter with huge crowds on platforms and terrible conditions for passengers inside packed trains."

Ms Doyle told the NSW Parliament last year some passengers had to stand for the distance between Blackheath or Katoomba and Penrith.

"Not only does this reflect poorly upon our public transport system for international and interstate visitors but also it drives local residents insane," she said.

She said overcrowding could be solved by running six- or eight-carriage trains on weekends, but the NSW government did not want to pay for additional staff.

A Transport for NSW spokesman said: "We know that our customers' needs are changing and acknowledge that more services on weekends are needed to meet demand into the future."

He also said the government was spending $1.5 billion to urgently increase capacity. New trains are expected on the Blue Mountains line from 2019.

"Transport for NSW regularly reviews Opal data to see where a boost in services is needed."

Other public transport to tourist destinations, such as ferries to Manly and buses to Bondi, also experience increased patronage outside of the weekday peak times.

Standing on the stairs on the 9:18 train from Central to the Blue Mountains last weekend. Photo: Michele Mossop

Mr Cronshaw said his customers had complained about the overcrowded trains - other tourism operators, including cafe and restaurant owners heard similar complaints.

He said the overcrowding was particularly bad during the April school holidays: "Over Easter they actually denied people from boarding on the train because it was so full."

"It's a regular occurrence unfortunately," said Eric Sward, the manager of The Mountain Heritage hotel in Katoomba. "The problem is obviously during the peak times mainly on the way up and on the way back and on the weekends."

Mr Sward said the overcrowding on trains could affect tourism to the Blue Mountains.

"It will over time if it continues," he said. "If it becomes common knowledge that 'Don't catch a train because you'll be standing up for two hours' then obviously people won't use it."

Tourism & Transport Forum chief executive Margy Osmond said: "There is no question that there are currently overcrowding issues on public transport routes to some of our world famous tourist destinations, such as the Blue Mountains.

"This is due to a number of factors, such as a significant rise in annual visitor numbers and the introduction of the $2.50 Sunday Opal cap, which has seen more Sydneysiders and visitors venture out to places like the Blue Mountains on the weekend."

Ms Osmond said increased patronage on trains was good news: "However, we don't want the Blue Mountains transport line to become a victim of its own success and become a deterrent to visitors or detract from the experience of those who make the trip."