What age would you let your child do this train by themself?

If you feel it’s necessary to make contact with the other parents so it’s not just her going off with a random then do that.
Aside from that teach her how to use the train and let her go if you think she will be able to manage it.
Age isn’t really relevant assuming she is 12/13 at least.

I had to start catching the tube to secondary school in year 7, with one change in between. I didn’t know anyone to start with so was doing this alone for the first few months at least. this was before smartphones! By 16 I would think most kids would be capable of catching a direct train.

By 16 we were chatting to the boys from other schools also on the same tubes and our parents certainly didn’t know them 😂

I think if your child is generally sensible it will be fine.

Do your suggestion if she is under 16. After that I would leave them to work it out themselves.

This - the boy you haven't seen is far more of a concern than the train journey.

With friends 14 I think. With someone you don't know, I wouldn't be keen, so certainly older.

I'd say train would be fine from age 11/12 if she got on the wrong train you felt confident she could a) call you and b) ask a train guard what to do

However going with a boy you've never met... 15/16 they could be anyone

Does the boy go to her school or some random of the internet

The boy is the variable - where does she know him from?

My 14 year old has been taking a similar length journey time wise, with one change (which sometimes has to be at a different point enroute due to cancelled/ delayed trains meaning missed connections) since he was 13 - he uses an app to check platforms and delays and alternative routes. The city he goes to is quite a bit bigger than York.

However he goes directly to a friend's house, I know the friend and the friend's parents and we did the journey together four times before he did it alone - the third and fourth times he navigated and I made clear I would just go with him even if he got on completely the wrong train, and he found the way competently.

He's dealt with getting on the wrong train home once and course correcting by getting off and catching a train back. He's also dealt with cancelled trains. Once early on he got lost walking back from his friend's to the station (in winter in the dark although it was only late afternoon) and his Google maps wasn't working, but luckily for some reason we could see where he was on location sharing and he rang us and we navigated him - so being on hand for phone support the first few times if necessary can be a good idea! That could have been hairy.

Just the train journey is fine at 13 IMO, in the daytime. Other variables might not be.

Train with friends you know 14/15. This situation with a random boy I would prefer not at all but at 17 you wouldn't have much say so I'd say 17.

I think you should let them do it alone. If they miss the train then they'll learn what to do. Wait for the next one. You physically being there won't actually make things any easier anyway. As you'll be 'hiding' in another carriage.

How old is dd? And how certain are you it's a boy she is meeting not some nonce she met online? Under 16 it would be a no from me. Over 16 I would travel with her /them then let them go shopping once you know he isn't a 40 odd year old bloke..

At 12yo my dd took a train from SE to NW and back again on her own. She had three changes each way, including on the way back having a train cancelled unexpectedly.
She was fine and loved it.

At that age what I'd say to her is I want a text to say she's there, and I expect her back by a certain time, and I want a text to say she's on the train home.

By secondary age they should be capable of doing that.
If they're year 6 I'd perhaps suggest they go by train and I'll drive them home.

I think your plan is a sensible one. Unless she's late teens then going on a long day trip with a boy she barely knows contains an element of risk. Catch the same train - different carriages - and vice versa on the way back. It would be different if it was a friend you know already, but in the interests of safety I think it makes perfect sense for you to travel on the same train but separately. If all goes well then she can have more freedom next time. It's not as if you would be following her around town. I think your plan is a fair compromise.

im 44, i still struggle with trains tbh.

i'd do your thing until they're both more experienced with getting on them.

Surely she could take her charged phone and call you if she gets in a muddle?

How old is she?

My daughter was taking an hour long train at 13 to visit a big city with friends. We’d trained her how to catch the train/find the station etc for a while before.

My issue wouldnt be the train, my issue is the boy she doesn’t know well wanted her to go 1.5hrs away. Why can’t they have a date locally?

I much prefer your solution this time.

I think the fact she doesn’t know him would be a big problem for me. Also 1.5 hours is quite far from home if something goes wrong, for me that would be 16 or even 17.
(and l live where kids walk to school from day 1)