The action centres on the company’s Passenger Assist service at Cardiff Central station and is being brought on behalf of 18-year-old Danny Thomas, an autistic man.
Mr Thomas relies on the scheme to travel safely from Swansea to Cardiff.
TfW has been accused of leaving him unattended on platforms, failing to meet him, or not providing the support he needs to board the train.
These issues have reportedly prevented Mr Thomas from attending more than 27 weeks of specialist drama training and education in Cardiff.
His mother, Sally Thomas, said: “After months of trying to communicate constructively with senior executives at Transport for Wales, Danny and other disabled passengers continue to be let down.
“This is not a one-off situation – Danny has been repeatedly failed.
“Through my advocacy, I know that he is far from alone in this.
“Many disabled passengers are facing the same barriers and the same failures.
“TfW must take accountability and make the changes needed so that disabled passengers can travel safely and with dignity.
“Until Passenger Assist works reliably for everyone across Wales, people like Danny will continue to be excluded.”
A Letter Before Claim has been submitted to TfW, which is the first step toward a judicial review.
Mr Thomas and his mother argue that TfW’s failures breach equality and human rights law.
He is represented by law firm Leigh Day.
Kate Egerton, solicitor at Leigh Day, said: “Danny is entitled to travel by train reliably and safely on an equal basis to everyone else.
“Our client’s experience paints a deeply troubling picture of a Passenger Assist service that exists on paper but is failing disabled passengers in practice.”
A TfW spokesman said: “We confirm that we have received a Letter Before Claim from Leigh Day on behalf of Mr Thomas and have responded.
“Given the legal nature of this matter, it would be inappropriate to comment further at this stage.
“We will continue to help disabled and older passengers travel on our trains, including through use of the Passenger Assist service.”
