Mr Williams told the broadcaster the role should not be performed by the Department for Transport, adding: “Someone needs to be accountable to the public.”
The Williams Rail Review was launched last September as a separate investigation into disruption following a timetable change in May suggested “nobody took charge”.
The Government said the review would be “the most significant since privatisation” and will consider all parts of the sector, including accountability, the franchising system and value for money for passengers and taxpayers.
Findings will be presented in a white paper in autumn, while reform is said to be scheduled for 2020.
Punctuality across Britain sank to a 13-year low in 2018, with one in seven trains delayed by at least five minutes as a series of major issues plagued the system.
Speaking earlier this year, Mr Williams said the DfT has taken on roles “it never intended to perform”, such as specifying which trains stop at which stations.
This will “have to change” if the railway is realigned to focus on passengers, he said, adding: “The levers to effect change do not come together coherently, and in too many instances only do so in the Department for Transport.
“A lesson we can learn from other countries is that some decisions are best taken by those closer to the detailed operations.”