The rush to purchase the online railcards caused the website to crash for several hours shortly after their release as it was unable to cope with the high demand.
The new cards, which will only be available digitally, will cost £30 for a year and are primarily meant to be used on smartphones.
To apply for the millennial railcard, firstly you will need to do so very quickly on the 26-30 Railcard website.
Chancellor Philip Hammond unveiled the scheme in last year's budget, during which he confirmed that the price-saving train fares would be available to anyone under the age of 30.
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.More news: The Celtics Are Reportedly Unsure Whether Marcus Smart Will Return This Season
Harriot Bishop wrote on Twitter: "Trying to get a 26-30 Railcard is about as painful as trying to get tickets for Take That's Progress tour in 2010 (and this is the least millennial reference you'll see on the subject today)". "Sort out your website to make it fair!"
There is now a limited number of cards available while the scheme is trialled to assess the impact on revenue and passenger numbers, however, it is expected it will eventually be rolled out across the UK.
In a message to the National Railcards Twitter account at 7.48am, youth worker Josh Booth, from Leeds, posted: "Been trying to get 26-30 railcard since 7am, your website crashing and been on hold on the phone since then". "We're really sorry this is happing and have our whole team on it".
Tom Drury, from London, wrote: 'The launch of the new railcard is a shambles'.
They will cost £30 and be valid for a year, and will be digital, kept on an app on the user's phone rather than as a physical card in a wallet.More news: Iraq lifts ban on intl flights to Kurdistan airports
The card must be downloaded on to a smartphone and is aimed at leisure travellers.
Tuesday's nationwide launch follows a trial across the Greater Anglia network, during which another 10,000 railcards were sold.
Currently, people aged 16-25 can buy a railcard to get a third off their fares, as well as couples in a Two Together scheme, or families in a group card.
An RDG spokesman said: "The trial of the 26-30 Railcard is part of the rail industry's long-term plan to change, improve and boost communities by enabling more people to travel by train".More news: Spurs sweat over Kane amid reports of lengthy absence
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