Rebecca Huitson, 12, underwent surgery at Newcastle's Royal Victoria Infirmary after she was dealt a "glancing blow" by an Edinburgh-to-London train.
Rebecca's arm was retrieved from the track after she was hit at Killingworth at 18:30 GMT on Monday, police said.
Michael Schenker, a consultant plastic surgeon, said there was a "small risk" the "replant" of her arm could fail.
He said Rebecca's arm had sustained considerable damage.Further operation
"I don't know how they found it but was told it was found quite far away from the patient," Mr Schenker said.
"It has a number of fractures so we have to deal with that at a later stage.
"The main thing was to get the blood supply into the arm as quickly as possible, and so far that is working."
Rebecca is expected to have a further operation.
"It is impossible to say at present what the final outcome will be, but we are working hard for her to have an arm with useful function in the end," Mr Schenker added.
Rebecca, who studies at Seaton Burn College, has now been returned to a normal ward from the ICU.Students 'distressed'
Principal Alison Shaw said everyone was hoping Rebecca would make a good recovery.
"A number of our young people were very distressed, when they heard what had happened," she said.
"Some of them had witnessed it, so we are trying to help them get back to their learning with appropriate support to deal with the trauma they have suffered."
A British Transport Police spokesman said that investigations were continuing but the incident appeared to be an "accident".
The Rail Accident Investigation Branch of the Department for Transport has been informed.