The Federal Government-owned Australian Rail Track Corporation's 2019 Hunter Valley Corridor Capacity Strategy released on Friday acknowledged there are currently more train sets in use "than the theoretical efficient fleet" leading to trains being "parked-up" along the world's largest coal export supply chain.

Capacity problems, exacerbated by recent derailments, have led to coal stockpiling at the giant Ulan, Moolarben and Wilpinjong mines, while Ulan owner Glencore has objected to a new Bylong coal mine because the single track Gulgong to Sandy Hollow rail line is already contracted beyond its theoretical capacity.

But ARTC proposals to spend hundreds of millions of dollars to increase capacity and productivity have been strongly criticised by environment and community groups after the strategy acknowledged a decline in prospective coal volumes to Newcastle Port terminals from 2023.

Contracted export coal volumes were 193.5 million tonnes per annum in the last three months of 2018 and remained "essentially stable at approximately this level until export volumes start to decline in 2024, falling to 152 million tonnes per annum in 2026 and holding constant from there", the strategy said.

Contracted volumes include up to 10.1 million tonnes per annum of domestic coal, with traffic from the Hunter to Central Coast power stations which "declines to 8.8 million tonnes per annum in 2021 and to zero in 2026".

The Hunter Valley Corridor Capacity Strategy outlines track investment options based on three coal volume scenarios - contracted, prospective and most likely.

Lock the Gate Alliance campaign coordinator Georgina Woods said the capacity strategy showed the Hunter and NSW faced "a fork in the road" in the next five years as global demand for coal slowed in response to global warming.

"According to the mining companies' committed contracts, export volumes of Hunter coal may begin to decline from 2023. Controversial coal mines like Watermark on the Liverpool Plains or the Bylong proposal are not yet locked in to contracts with the rail operator," Ms Woods said.

"We can invest the money ARTC is proposing to waste on hastening coal exports or create new jobs that are not at risk of vanishing when demand for Hunter coal declines."

The ARTC 2019 strategy document detailed two new projects - a real time data process allowing digital train planning, and a train-to-train communications-based system that will allow trains to run closer together without fixed signals beside the track. The system also enables automatic braking of a train.

The combination of both systems "has the ability to significantly reduce direct human intervention in train operations", ARTC said.

About 66 loaded trains per day, or one train every 22 minutes, is required to meet the mining industry's 2019 contracted volumes using trains at the Hunter coal train length limit of 1.543 kilometres, the strategy said.

Train lengths are restricted because of departure road and line loop lengths and standing distances between signals and level crossings.

But ARTC said it was reviewing longer trains and has started engineering investigations because increasing train length was "a potentially effective mechanism to increase capacity in a systematic manner".

It remained "cautious" about overlength trains because without supporting infrastructure investment "the de facto priority it gives these trains, the constraints on where they can cross other overlength trains, and the limitations they place on the system generally, means that they are likely to lead to a net reduction in system capacity".

ARTC did not anticipate allowing longer trains on the single track line servicing Ulan, Moolarben and Wilpinjong mines, despite calls for longer trains from mine operators, after modelling showed longer trains decreased capacity.

Minutes of the Moolarben mine community consultative committee meeting in March showed a recent train derailment near Denman left the mine with full coal stockpiles after the line was closed.

The minutes show a mine representative was asked whether rail problems "would have any impact on employment" but was advised it was "unlikely" to do so.

The ARTC 2019 corridor strategy showed the company is also considering improving capacity on the single track sections by increasing empty train speeds to 100 kilometres per hour.

ARTC is also considering work to improve congestion problems at the Muswellbrook rail junction "bottleneck" after analysis showed "operating practices" allowing multiple trains to leave the port terminals caused queues at Muswellbrook.

The junction "stands apart from the other junctions due to the need to sequence trains onto two single track lines and the significant number of trains from both lines, which means a large number of conflicting movements", the strategy said.

It noted the Muswellbrook to Newcastle Port terminals was the core of the Hunter network and "coal dominates operations across this corridor".

"The passenger services, which get priority and run down the coal services, create a disproportionate loss of capacity, particularly in the loaded direction," ARTC noted.

The strategy noted six rail bridges would be fitted with monitoring equipment and another 22 would be inspected and subject to further recommendations after a 2017 review found significant bridge defects in the system, where some steel bridges are about 100 years old.

The strategy also noted the need for regular work below rail lines due to earthworks failures.

"The majority of the Hunter Valley rail network is built on an earthworks formation which was constructed during the early 1900s. The running of 30 tonne axle load rolling stock would not have been envisaged by design work done during this period," ARTC said.

A spokesman for Glencore said the company's position on rail network capacity "hasn't shifted" since its submission to the Independent Planning Commission in 2018 on the proposed Bylong coal mine. It had not made a submission to the ARTC on the 2019 corridor strategy.

A NSW Minerals Council spokesman said the council did not make a submission to the strategy.