The 26-30 Railcard goes on general sale at noon.
The Rail Delivery Group (RDG), which is promoting the card, said: “Research revealed that those under 30 were the only age group to have less income now than they did 10 years ago.
“All 26- to 30-year-olds will be eligible to save an average £125 a year on Off-Peak and Advance rail journeys.”
The rail discount card was first promised by the chancellor in his Budget in 2017. It extends most 16-25 railcard privileges to people aged up to 30, and has been popularly known as the “Millennial Railcard”.
But the rollout has been slow and frustrating for many in the age range.
A pilot programme for the card began in the Greater Anglia area in December 2017, with just 10,000 on offer. All had been sold within two months.
In March 2018, the railcard went on sale nationwide – but with only 10,000 more available, one for every 500 people in the 26-30 cohort.
As many in the eligible age range of 5 million tried to obtain the discount card, the website crashed.
The card is available in digital form and costs £30 for a year. It can be bought up to the day before a person turns 31.
Since Philip Hammond announced the card, around one million people have had their 31st birthday and are therefore too old to qualify.
Someone born on New Year’s Day 1988 must wait until the same date in 2048 before they can buy a Senior Railcard. Alternatively, they could fulfil the requirements for a Family & Friends or Two Together Railcard.
The card goes on sale on the day that rail fares went up by an average of 3.1 per cent.
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Jacqueline Starr, managing director of customer experience for the RDG, said: “The launch of the National Rail 26-30 Railcard marks a key milestone in the rail industry’s commitment to boost local communities by making rail travel more accessible.”
Also today, the transport secretary Chris Grayling has announced a new 16 & 17 Railcard, which will extend the opportunity for half-price child fares for up to two years.
Mr Grayling said: “Today’s announcement of a new 16 & 17 Railcard could cut the cost of travel by hundreds of pounds a year for young people and their parents, making it cheaper to get to school, college and work.”
The card will be launched in September. It effectively deepens the one-third discount currently offered by the 16-25 Railcard, and is likely to have fewer restrictions on when cheap fares are available.