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.