The incident happened on Wednesday after the match at Kingsholm between Scotland and Japan.
One departure was delayed by about 20 minutes while a new train manager was brought in to replace the one who was injured, a Great Western Railway (GWR) spokesman said.
Many fans complained of overcrowding on trains and poor communication.
GWR spokesman Dan Panes said there was an incident "between a member of the public and a member of staff".
"As a result that member of staff couldn't work on the train and we had to get a replacement in, which took around 20 to 25 minutes."
Mr Panes said GWR put on four additional trains and "strengthened" existing trains due to the high number of passengers travelling.
"It's very, very difficult to get more than 2,500 people onto trains almost instantaneously," he said.
"We had plenty of capacity but we could provide all the trains that we have in the fleet just for Gloucester and it would still take some time to load that number of people on board those trains."
The BBC's head of sport for English Regions, Charles Runcie, who was at Gloucester station after the game, said there was "an almighty scrum" to get on a two-carriage train just before 18:00 BST.
"Word filtered through that the driver was refusing to take the train out," he said.
"GWR have had three three years to prepare for this and they still put on the same little carriage. There was no communication. It was just rank awful."
There were also complaints about overcrowding in Cardiff after Australia's match against Fiji on Wednesday, and at the weekend.
A train manager was injured in an assault on a platform at Gloucester station after a Rugby World Cup match.