In which case maybe the Guard should have gone down the train with her and found her another space, unless all the disabled space on a reasonably long train were filled at 11am on a Sunday morning.
It was the first train from Plymouth towards Totnes and onwards due to overrunning engineering works.... giving GWR an added excuse to their lack of staff
https://www.gov.uk/definition-of-disability-under-equality-act-2010
So being unable to hold a baby due to a temporary injury would certainly not make the woman disabled.
We don't know it was a temporary injury. We just know It was an arm injury. we don't know if it was a long term injury.