It's supposed to be Sydney's third city, but residents say they must endure lengthy commutes because of a lack of express trains.

The Mayor of Liverpool, in Sydney's south-west, said residents "deserve" express trains to cut down lengthy commutes into the CBD.

Liverpool's train station is just 26 kilometres from Central Station, but the area does not have an express service.

On a good day with no delays, the journey into Sydney's CBD takes just under an hour. 

Liverpool has been dubbed Sydney's third CBD after Sydney central and Parramatta.

"With the amount of population that get on the trains and travel away to get to work, I think we deserve fast trains in this area," Liverpool Mayor Wendy Waller said.

She said the train trip now takes longer than when she was a child growing up in the area.

The situation stands in stark contrast to other populated areas in Sydney's west, like Parramatta where up to 20 express trains run between Parramatta and Central in peak times.

The ABC asked commuters at Liverpool station for their thoughts.

"I think we should have an express train to Liverpool, everyone works in the city and that and it would be a good idea," one woman said. 

"If you got an appointment and you're running late, you're not going to make it," a man said. 

"I go to the city for work every day, so that would cut off like an hour from my journey," said another. 

Transport experts said a fast train service could be started without changes to existing infrastructure.

"There should be both express services and all station services to serve the commuters of south-west Sydney," public transport advocate Roydon Ng said.

Mr Ng said express services began disappearing in 2013 and were completely gone by 2017 when they disappeared on the T2 line.

But Transport for NSW said there is no room on the rail network for a Liverpool express.

"The reason why we couldn't get very fast trains from Liverpool into the CBD, is there are lots of other trains out there," chief operations officer Howard Collins said.

"In the last few years, services have gone up by 20 per cent, in the peak you can get a train every five minutes, off-peak every 10.

"But it is a little bit slower, and that's because we have so many other trains going around the network," he said. 

Mr Collins hinted that could change when new infrastructure like the new Sydney Metro line starts operating.

The Sydney Metro is Australia's biggest public transport project and the western arm will link the CBD to Bankstown with an expected opening of 2024.