Josephine, Toitu's double-ended Fairlie steam locomotive, notched up 147 years on Labour Day.
To mark the occasion, the museum threw a birthday party, with face painting, balloon art, and the obligatory cake.
Volunteers from Otago Girls' High School also dressed up in Victorian clothing in a nod to Josephine's heritage.
Toitu visitor programme co-ordinator Phoebe Thompson said the museum celebrated Josephine's birthday every year, and would be planning something extra special for her 150th in 2022.
There were train enthusiasts who turned out every year to pay homage, she said.
People had been lined up outside the door before celebrations got under way.
One of those was June Yoo, who was visiting Dunedin from Auckland with her daughter, Sarah.
She said they wanted to visit the museum, and were excited to check out the festivities.
Invented by Scottish engineer Robert Fairlie, the first successful double-ended locomotive, called Little Wonder was demonstrated on the Ffestiniog Railway, in Wales, in 1870.
Josephine was ordered for use on the newly built Dunedin and Port Chalmers railway in 1872.
She first raised steam on September 11 that year, and lasted in service until 1900.
She was later used in the construction of railway lines, before her retirement in 1917.
