The managing director of rail franchise First Great Western (FGW), Alison Forster, has been moved to a new post after the company's poor performance.
A rail watchdog has called services on First Great Western "abysmal"
She has presided over poor punctuality and a row about overcrowding, which led to a fares strike by a small group of passengers.
She will now become Rail Safety and Performance Director at First Group.
Andrew Haines, the group's UK Rail managing director, will take over daily operations at First Great Western.
Earlier this year a rail watchdog had called for the government to review the FGW franchise, saying it was an "abysmal service".
London TravelWatch, in its plea to the government this summer, cited late trains, overcrowding and poor customer care.
Earlier this year FGW said it was working to improve its service, but that it was not breaching franchise conditions.
And in June a spokesman for FGW said it was "aware our performance has been poor and we have already apologised for this".
Moir Lockhead, FirstGroup's Chief Executive, said: "In recent months Alison and her team have been addressing the challenges faced by the new franchise and their efforts have resulted in improvements in our performance."
He also said that Mr Haines had helped deliver "sustained improvements" at First TransPennine Express, First ScotRail and First Capital Connect, the group's newest franchise.
"At each franchise punctuality and reliability has reached over 90% in recent months," said Mr Lockhead.
He also said that FGW and Network Rail were investing nearly £1bn to improve overall performance in the region.