But it wasn't enough to derail the carefully laid plans of the museum's army of volunteers - they pressed ahead at full steam.

"It's not the way we wanted to start but it could have been a lot worse," museum director John Cheeseman said.

It's been a long journey to get the rail yard open to the public again after bankruptcy and then a dramatic forced liquidation closed its doors three years ago.

In the end, fire crews were first through the gates on Sunday - responding about 6am to reports of a carriage alight in the yard.

While they managed to stop the blaze spreading, the first carriage - a privately owned machine from the 1930s - was completely gutted. Two others nearby, including a model from the 1950s housing some of the museum's spare parts, were also badly damaged. ACT Police said they are now investigating the fire and have called for witnesses to come forward.

But Mr Cheeseman said the little museum that has long been dogged by set backs got lucky that morning too - the blaze was lit in the storage yard and did not affect its display cars, where security has been ramped up in recent months.

It appeared someone had broken into the yard to light the fire, he said.

"We had a break in just last week but this is the first time something like this has happened," he said. "We do get a bit of graffiti. It's a particular shame for the owner of the 1930s carriage."

By 10am, the first crowds were already streaming through the museum's doors as the team saw off investigators.

Not one to skimp on hospitality, volunteer Glenn Bridgart offered this reporter a sneak peak at the historical machines in its collection - including the steam engine that hauled the very first train into Canberra in May 1914. Two years ago, thieves stole crucial parts from the locomotive which left it inoperable.

Now under new management, the museum's collection was smaller than before but Mr Cheeseman said the volunteer group had managed to save the most important pieces from liquidation. About a third of its exhibits remain after auctioneers sold off scrap metal and machines in 2017.

The museum closed the year before when its freight operation - intended to subsidise expensive heritage train trips - failed owing $700,000.

Mr Cheeseman said he hoped the museum would again tell the story of how rail shaped the fledgling national capital - back before the rise of the car. Old tuckerboxes, lanterns and other precious scraps of history are also on display.

But volunteers warn the legends of an old machine still buried under Garema Place in the city are the stuff of fiction.

"There were trains, light rail, that went right through to Parliament House and even Civic when the city was being built though," Mr Bridgart said. "The story is a train got stranded when the line ended and they buried it there [in Garema]. But that one's just a story."

While its heritage train trips are not back up and running yet, the museum is working to relaunch them next year.

On Sunday afternoon, as it's first day back in business came to a close, Mr Bridgart said the museum had counted about 130 visitors.

"We're happy with that but we could of course do with more."

Anyone with information about the fire should contact Crime Stoppers 1800 333 000, quoting reference number 6445919.