The Nationals MP said in February the NSW government would launch up to three extra daily return trains between Singleton and Newcastle in early 2020.

It came after more than a decade of campaigning from public transport advocacy group Two More Trains For Singleton, which had been pushing for additional day-time services.

Mr Johnsen's announcement a month out from the state election also included the chance for a Singleton-bound train leaving Newcastle at night.

At present, the last train to Singleton, and stations west of Telarah - Lochinvar, Greta and Branxton - departs Newcastle at 5.59pm each weekday and at 5.52pm on weekends.

The next train is not until after 3am every day.

Asked this week when the number of services would be confirmed, and timetables announced, Transport for NSW was scant on details.

"Planning work to deliver extra return train services early next year between Singleton and Newcastle is well underway," a spokesperson said.

"Adding extra passenger services is a complex process and we're currently in close consultation with track owners and freight operators who share this busy rail corridor to get these additional passenger services up and running."

While negotiations are understood to have progressed to the point of slots being pencilled in for at least two of the return trains to run, the Rail, Tram and Bus Union has questioned how Transport for NSW will implement the services.

"We have no idea how the NSW government intends to source or staff these additional train services on the Hunter line," the RTBU's NSW secretary Alex Claassens said.

"They can't poach existing Explorer and Endeavour [class] trains as these services are currently stretched to their limits. Moreover, there are already significant staff shortages."

Transport for NSW and the Australian Rail Track Corporation, which manages the heavily coal-train occupied line, have previously clashed about freeing up slots for passenger trains.

Mr Johnsen said he was not aware of any issues regarding the delivery of extra services on the line.

"Whilst I think we can all understand these matters can be complex, I have no reason to believe that the services that have been committed are anything but on schedule for early next year," he said.

Two More Trains For Singleton spokesman Martin Fallding said he had been in talks with Transport for NSW about a community meeting and he was of the understanding the plans were on track.

Singleton mayor Sue Moore said an update should have been provided to the community by now given it was nearing the end of the year.

She said for the use-it or lose-it services to be successful there needed to be a long period of promotion beforehand to ensure the community was well aware of the additional trains.