The Lost One

before Big One came to be there was another identical engine at the head of the SDF.
It was also called Big One, but called that in respect of her wishes to not be called big boy.
Up until 8001 we know arrived, this older engine numbered 4001 saved countless lives and rescued many stranded trains.
Her last run was a permanent transfer to earth where her cars were handed over and she was shunted into a siding and forgotten.
Years later a teenager stumbles across her and decides to help.