Extra trains will be run between Sydney and the Blue Mountains to reduce severe overcrowding in carriages on weekends and allay fears some tourists will stop visiting the world-heritage area.

Bowing to pressure from tourism operators and local residents, the state government has announced plans for 24 new express train services between Sydney's Central Station and Katoomba in the Blue Mountains on weekends from late this year.

The express trains will operate eight-carriage sets – double the size of those running at present – and cut the travel times for a return journey by about 20 minutes to four hours.

The increase equates to about 25,000 additional seats each week, including an extra 18,000 on weekends. A new return express service between Lithgow and Sydney will also run on weekdays.

Tourism operators have been concerned that crowded trains and "absolutely disgraceful" toilets on services will deter visitors from the Blue Mountains.

Some weekend services have been operating at more than 150 per cent capacity, forcing many passengers to stand for long periods of their journeys.

The situation has been worst on Sundays, when large numbers of people take advantage of a $2.50 cap on travel using their Opal cards.

Jason Cronshaw, the managing director of Fantastic Aussie Tours, which operates the Blue Mountains Explorer Bus, said the extra trains would go a long way to reducing the likelihood of crowding.

"It will mean that pressure can be dispersed across more services," he said on Wednesday. "I was shocked [at the latest announcement] – I didn't think it would be that big."

Transport Minister Andrew Constance said the government would also take immediate action to ease the pressure by doubling capacity on six weekend services to the Blue Mountains in the first week of July from four to eight carriages.

"We know many weekend trains are packed to the brim so we're taking action right now," he said.

Sydney Trains chief executive Howard Collins said the trains to and from the Blue Mountains had been "packed to the gunwhales" and the extra services would help reduce the overcrowding in carriages for tourists and commuters alike.

"I've used them three or four times in the last few weeks and everyone is standing. Doubling the length of the train, giving that capacity that we really need, will make a massive difference," he said.

Mr Collins said the government's plans to spend $2.3 billion on a new fleet of intercity trains would also help improve services from 2019.

However, before the first of those new trains can start running to the Blue Mountains, station platforms and rail lines will have to be modified because they are too wide to make trips beyond Springwood to Katoomba and Lithgow.

"We are working with government and our engineers to find world-beating technologies so we can [run the new trains to Lithgow]," Mr Collins said.

"We are keen to do this as quickly as possible because the demand is there."