"I have been given the additional charge of the minister of state of railways," Roy told newsmen after completing the formalities of the takeover at the Eastern Railways headquarters here.
He said Banerjee''s tenure was the "golden era" of the Indian Railways in terms of induction of new technology and introduction of new projects.
"I will work to turn Mamata Banerjee''s ''vision 20:20'' into reality and implement with the railway team the projects she had announced in the railway budget," Roy said.
PMO sources said in Delhi that the Railway portfolio will be held at the Cabinet level by Prime Minister Manmohan Singh who has accepted Banerjee''s resignation with immediate effect.
Roy, who is also Minister of State for Shipping, will continue in the post while holding additional charge as the MoS Railways.
The Railway Ministry already has two Ministers of State - Bharatsinh Solanki and K H Muniappa.
Banerjee, who will be sworn in as the Chief Minister of West Bengal tomorrow, had sent in her resignation to the Prime Minister earlier in the day.