"Everyone was terribly excited -- we were all so sick of that long journey up the incline and it was the start of a new era," Mrs Flynn said.
"I remember my husband saying, 'we're never going to see another steam train in Featherston again; they'll all be diesel from now on'."
The Rimutaka Tunnel was officially opened on November 3, 1955, to replace the difficult-to-navigate Rimutaka Incline railway.
The Rimutaka Incline, a 4.8 km stretch on a treacherously steep gradient, had been running since the 1870s, with Fell Engines pulling the trains between Summit and Cross Creek stations.
At the time, Mrs Flynn said, a train journey between Featherston and Wellington took around three hours and 20 minutes.
"It used to just crawl up the hill -- it would get very smoky as well."
This continued until the 1930s, when the then New Zealand Railways Department deemed the incline "extremely costly", and began making plans for an under hill tunnel.
The project was delayed thanks to the breakout of World War II, but work on the tunnel began in earnest in 1951, costing around 3.2 million pounds to complete.
Mrs Flynn has kept the Times-Age articles covering the opening, which describe an animated crowd of about 5000 waiting for Prime Minister Sidney Holland to cut "the special blue ribbon".
Wairarapa MP Bertie Cooksley told reporters, "there are no words which will adequately describe our feelings that at last what was hope is now a reality".
As a pleasant surprise, the now obsolete Engine H199 was gifted to the Featherston community by Minister of Railways John McAlpine.
The open day will also be paying tribute to the crew who drove HI99 on its final journey between Cross Creek and Summit on October 29, 1955.
"That day, they took the engine up to summit, and didn't come down for several hours," Mrs Flynn said.
"I think there was quite a party."
The Fell Museum has hosted several reunions for the crew, but most have since died -- save for firemen Ron Eustace and Bill Rossiter.
Both live outside the district -- Mr Eustace is still driving trains in Western Australia -- but call in to the museum when in town.
For more information about the open day, contact the Fell Museum on 06 308 9379.