Irish immigrants will be remembered at an event in the US state of Utah celebrating the 150th anniversary of the 'Golden Spike', the moment when the First Transcontinental Railroad was completed.
The railway connected the east and west coasts of the United States and was finished on 10 May 1869.
An estimated 10,000 Irish immigrants worked on the railroad.
Many of them were veterans of the US Civil War and made up at least half of the workers from the Union Pacific railway company.
They worked alongside a mix of Mormons, other European immigrants and recently emancipated African Americans to construct the line from the East.
Many Irish also worked alongside the 15,000 Chinese immigrants who were the main workforce building the line from Sacramento, California to Promontory Summit, Utah.
The ceremony in Utah today will be addressed by Irish Ambassador to the US Daniel Mulhall.
The festivities will feature full-size working replicas of the two steam engines that faced each other, nose to nose, in an iconic photograph taken of a celebration held the day the cross-country rail line was completed.
The picture captures throngs of bearded crewmen toasting the occasion with bottles of whiskey as they clamored around the two engines, Central Pacific's No 60 Jupiter and Union Pacific's No 119.
Speaking ahead of the event, Ambassador Mulhall said he was proud and honoured to mark the enormous role played by Irish immigrants in the building of the transcontinental railroad.
"Theirs was a magnificent contribution to the making of modern America," he said.
The US President Donald Trump today paid tribute to the workers who built the transcontinental railroad.
In a statement he said: "We honour the undaunted efforts of the people who made this innovation possible, including workers of Irish and Chinese descent, as well as freed slaves, Native Americans, Civil War veterans, and Latter-day Saints."
Pulitzer Prize-winning historian Jon Meacham will give a keynote address and another highlight of the commemorations will be a costumed re-enactment of 1869's ceremonial driving of the last spike, cast in 17.6-karat gold, that connected the finished rail line more than 2,816km between Sacramento, California, and Council Bluffs, Iowa.
The achievement was announced by telegraph, flashing the single-word message "Done" across the country in what was widely regarded as one of the nation's first media events.
About 20,000 people are expected to attend today's events, organisers said.
The commemorative golden spike, which was immediately replaced by an ordinary iron spike in 1869, is currently on display with related artifacts at the Utah State Capitol in Salt Lake City.
Construction of the railway, which cut US coast-to-coast travel time from many months to just a week, greatly accelerated Anglo-European settlement of the American West and aligned it politically with the Union states of the North.
It also hastened the demise of the Plains Indians, as well as the bison herds on which they depended.
An anti-Chinese backlash following completion of the railroad led to passage of the federal Chinese Exclusion Act of 1882, marking the first significant law restricting US immigration. Those restrictions remained on books until 1943.
The Chinese worked for less pay for longer hours than their white counterparts, and performed the bulk of the most dangeroustasks. Untold numbers - as many as 1,200 by some estimates -perished in blasting accidents, snowslides, falls and other mishaps.
Stymied by a lack of records from that era, many Chinese-Americans have only managed to trace their family's roots to the Transcontinental Railroad in recent years, with the help of research organised by Stanford University in California.
Additional Reporting Reuters