They also reassured customers at 10:20am that there were still railcards available, however there were still website difficulties due to the demand.
The Government promised that up to 4.5million Brits aged 26-30 will benefit from the card, but 10,000 "millennial" railcards were made available today as part of a trial of the scheme.
Many people turned to Twitter to share their frustration at being unable to get on the website, with #railcard trending on the social networking site.
Demand for the railcard has been high since it went on sale this morning, with the official website crashing under the strain.More news: Barbershop study trimmed black men's hair and blood pressure (copy)
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. Sort out your website to make it fair!"
Meanwhile, Tom Drury, from London, wrote: "The launch of the new railcard is a shambles".
However, rather than being a physical card, the 26-30 card will be available via the Railcard app on your phone. There will be a £12 minimum on tickets between 4.30am and 10am, excluding weekends and public holidays. "We're really sorry this is happing and have our whole team on it".
The site, railcard.co.uk, showed a message saying that "the service is unavailable" after a scramble for the new tickets for 26 to 30-year-olds.More news: Probation for student who rubbed tampon blood on Jamaican's bag
The wider rollout today follows a trial on the Greater Anglia network, and anyone applying for the railcard will need to pay by card, have proof of their age either by a driving licence or passport, and have a passport-style picture on hand.
Chancellor Philip Hammond said in his Budget speech in November that the railcards would give "4.5 million more young people a third off their rail fares".
There was previously no nationwide railcard available for people travelling alone between the ages of 26 and 59 unless they were disabled or in the armed forces.More news: Taro Daniel took advantage of Novak Djokovic not being at his best