The Rail Maritime and Transport (RMT) union said SWR's nationalisation was "a major step forward and is a clear rejection of the failed privatisation model".
But its general secretary Eddie Dempsey expressed concern over outsourcing staffing for security, cleaning and other areas to private companies, saying: "The job is incomplete when our contracted-out members remain outsourced and not reaping the benefits of nationalisation."
Seven more companies will be renationalised by 2027 as each of their contracts end – or sooner if their performance is judged to be unacceptable.
These are:
Rail services in Wales were nationalised in 2021. Scotland took trains into public ownership the following year.
GBR will manage rail tracks across England, Wales and Scotland and operate most passenger train companies in England.