A railcard for people aged between 26 and 30 will go on general sale on January 2.
The discount cards sold out within hours when 10,000 were made available in March as part of a trial.
After the impact on revenue and passenger numbers was assessed, the so-called millennial railcard is being made available for all 4.5 million Britons aged 26-30.
Industry body the Rail Delivery Group (RDG) said last month the roll-out would take place before the end of the year, but this date has been pushed back to January 2, coinciding with the annual fare increase.
The delay means people born on January 1 or 2 1988 will be unable to purchase the card before becoming ineligible due to being too old.
One of those affected is Clint Furnish, from Crawley, West Sussex, who turns 31 on New Year’s Day.
He had planned to use the railcard to save money on trips to visit his partner in Stoke and believes the delayed roll-out will cost him around £450.
“The delay to the railcard’s release means I’ll miss out,” he told MoneySavingExpert.com, adding: “I wouldn’t mind if they had said 2 January all along – but when you’re promised something and they change that, it’s not fair.”
An RDG spokeswoman said: “We intended to launch the 26-30 Railcard by the end of the year.
“Given how timings have worked out, this would mean launching it over a bank holiday.
“When the 26-30 Railcard goes on sale we want to be able to provide the best possible experience to our customers, which is why it will go on sale at midday on January 2.”
The cards cost £30 each year and save a third off most fares.
There are no discounts on season tickets and a £12 minimum fare applies to all journeys between 4.30am and 10am, excluding weekends and public holidays.
The card must be downloaded on to a smartphone and is aimed at leisure travellers.
It is the first nationwide railcard available for people travelling alone who are aged 26-59 and not disabled or in the armed forces.
The 26-30 Railcard was initially piloted in East Anglia before the nationwide trial in March.
Cardholders save an average of £125 a year, according to the RDG.