The cards could be applied for from today but as there are only 10,000 now available, they are being issued on a first-come, first-serve basis, the Rail Delivery Group (RDG) said.
Tom Drury, from London, wrote: "The launch of the new railcard is a shambles".
There are 10,000 of the 26 to 30 railcards now available on a first-come, first-served bases.
"It's a great idea for the rail card to be available on your phone, rather than as a physical card - I can imagine this being a hit with this age group and it's much better for the environment too".
Anthony Smith, chief executive of the 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".
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Passengers across Britain aged 26-30 are eligible for the discount card for the first time.
After the price hikes to rail fares in January, a 26-30 rail card could ease some financial pressure from millennials, especially those who rely on trains on a daily basis.
"Research being gathered as part of the 26-30 Railcard trial is being used to inform discussions with the Government about a national roll-out, and to develop new products that make leisure travel easier and better value for customers". However, the website was still down as of 2pm on 13th March. The railcard must also be paid for by card. It can only be bought online and must be downloaded to a smartphone.
This is slightly more nuanced than it first appears.
Despite these restrictions, if you do regularly travel off peak or over long distances, the new railcard could save you a packet.
Not only that, the railcard will be valid for the full year until you turn 32! Meanwhile, purchasing the railcard over the phone is also proving hard - The Independent has been on hold for more than 40 minutes at the time of writing (with no end in sight).
Rail Delivery Group, the company that is releasing the millennial railcards, has yet to specify when they will be fully rolled out across the UK.