I would be concerned but her parents know her best. The trouble with trains is that you can never totally rely on them to get anywhere on time. What if there’s a cancellation or the train is halted then a bus replacement.
My ex husband asked for our son, then 14, to travel from Wiltshire to North London. Changes. Were London Paddington, on the tube to Kings Cross the overground for two stops. I said no, please meet him at Paddington which he did. Unbeknown to me the return journey he just put him on the train in North London and he made his way to Wiltshire. My. Son did well and grew in confidence the more he did the journey, although it terrified me each time. He had a mobile which was good and at that time was over 6 foot so looked older. He now has travelled the world. For my daughter I don’t think I would have been happy at that age. There’s a difference between boys and girls and how safe they maybe. It shouldn’t be that way but there is. Untimely does your GD feel happy doing it? Maybe offer to go with her and travel back straight away. It’s a long journey if she’s not done it before.