The T4 loader being planned by the Rio Tinto and Xstrata-backed Port Waratah Coal Services (PWCS) could add another 140million tonnes a year to the Hunter’s export coal capacity.

Although a final size for the loader is yet to be decided, documents lodged with the NSW Department of Planning put an indicative value on the project of $3.5billion.

By comparison, stage one of the recently completed Newcastle Coal Infrastructure Group terminal cost about $1billion.

Capacity along the Hunter Valley ‘‘coal chain’’ between the mines and the port is limited by the weakest link in the chain.

The Australian Rail Track Corporation (ARTC) is building a third line between Minimbah, near Singleton, and Maitland, and a second rail bridge onto Kooragang Island is being planned to handle the traffic in and out of T4.

This means the weakest link in the system is likely to be the two coal lines between Maitland and Sandgate unless coal trains can use the two adjacent dedicated passenger lines or unless ARTC builds a third coal line.

While details were not available at the time of writing, an ARTC spokesman said it made sense to maximise use of existing rail lines before incurring the cost of a third line.

But the spokesman emphasised, however, that passenger services retained priority over coal services under the conditions of ARTC’s operating lease.

A PWCS spokesman said various rail options on Kooragang were being considered for T4 but the configuration of lines over the Hunter River from the main northern line was a matter for ARTC.

Public transport advocate Darrell Harris said putting coal trains on passenger lines would put pressure on passenger services.

‘‘Whether they want to acknowledge it or not, passenger numbers on the Hunter rail service have been growing by about 8per cent a year for the past three years and occupying passenger tracks with coal trains makes it harder to cope with further expansion,’’ he said.