A nationwide trial of the so-called millennial railcard begins on Tuesday but card numbers will be limited.
The one-year railcard for 26 to 30-year-olds will cost £30 and offer a third off most fares in England, Wales and Scotland.
Only 10,000 will be released – enough for one in 500 of the eligible population – and the cost of some peak time travel will not be reduced.
The nationwide launch follows a trial across the Greater Anglia network.
Another 10,000 railcards were sold during that trial.
The new discount card was announced in Chancellor Philip Hammond’s Budget in November, but received a mixed response from its target audience.
Railcards were first introduced as a way for train companies to help fill seats during off-peak times. The card for 16 to 25-year-olds has existed in one form or another since 1974.
Anyone applying for the railcard will need to pay by card, have a proof of age, either through a driving licence or passport, and have a digital, passport-style photo.
Applications should be made on the 26-30 railcard website, and holders will need to download an app before they can use the card.
The discount for some during the morning commute is limited as a minimum fare of £12 applies to all journeys made before 10am, Monday to Friday.
The Rail Delivery Group said discounts were not given on season tickets, but a weekly or monthly season ticket could still be the cheapest option for those in this age bracket who commuted frequently by train.
Anthony Smith, chief executive of watchdog Transport Focus, said: “The nationwide trial of the 26-30 railcard is welcome news for hard-pressed passengers and will help take some of the sting out of rising rail fares.
“With less than half of passengers satisfied with the value for money of their journey and even fewer young people satisfied, this will help make travelling by rail that little bit more affordable for young people.”