Volunteers from Transport Heritage NSW who are revitalising the famous steamer reached a significant milestone at the weekend when they completed a steam test in the locomotive's restored boiler.

The test was the first time 3801's boiler had been fired up in more than a decade, with its original full working pressure restored to 245 psi.

Transport Heritage NSW volunteers have been quietly making headway on the restoration over the past few months.

The historic locomotive has been out of action since 2007, when it was withdrawn for an overhaul, and is now being worked on by a small army of volunteers at Chullora Railway Workshops in Sydney.

Transport Heritage NSW, the not-for-profit organisation the state government established to manage state-owned heritage rail assets in 2013, lists the 3801 restoration as one of its priority projects.

Transport Heritage NSW chief executive Andrew Moritz told Topics there was 20 active volunteers involved with the project, supported by three paid staff and a project director on secondment from Sydney Trains.

He said interest in the project was growing with each milestone achieved in the restoration.

"No other steam engine in Australia has captured the imagination of the public as 3801," Mr Moritz said on Tuesday.

The 3801 famously set the record for the fastest journey by rail from Sydney to Newcastle on June 28, 1964.

The non-stop train took two hours, one minute and 51 seconds to reach the now defunct Newcastle railway station from Central.

The time was later beaten by an XPT, but it is one that travellers of today would long for.

Asked what made 3801 so special, Mr Moritz said it was the locomotive's power, reliability and streamlined shape.

"When it was first launched in 1943, the 38s instantly changed the image of the NSW Railways, a bit like the XPTs did in the 1980s," he said.

"Locomotive 3801 was the first of its class. Its art-deco style streamlining combined with its power and reliability, demonstrated the NSW Railways' ability to keep up with the rest of the world.

"In preservation, 3801 became famous for being the only steam loco to have travelled to all mainland Australian states and territories."

The 3801 last visited Newcastle in 2007. It had made regular appearances at Hunter Valley Steamfest over the years.

As for a possible return to the Hunter, Mr Moritz said Transport Heritage NSW was working with relevant stakeholders to plan a program for the locomotive's comeback.

"Right across the organisation, anticipation is building to see 3801 back in steam," he said.

"At this current stage in the project, we're confident of relaunching locomotive 3801 in the first half of 2020. [We're] currently in the early stages of developing a relaunch program of events."

Hunter Valley Steamfest in April too soon?