Welcome, Helen. I remember reading the following words in Chapter 6 of
The Sea of Adventure, and the chapter is indeed called 'Travelling Far'. It's not a Famous Five story but it's a thrilling adventure story (one of my absolute favourite Enid Blyton titles) about Jack, Lucy-Ann, Philip and Dinah, who visit some remote islands in Scotland. It's possible that that's the book your partner remembers - unless there's also a "travelling far" passage in a Famous Five book!
It really was very exciting to go to bed in a train. It was queer undressing with the train swaying about, rushing through the night at sixty miles an hour.
It was nice to be in bed, listening to the 'tutta-tut-tah! tutta-tut-tah!' of the train wheels turning rapidly over the rails.
'Travelling far, travelling far, travelling far,' said the wheels to Lucy-Ann, as her eyes closed, and her mind swung towards sleep. 'Travelling far...'
In spite of all the excitement the four children were soon fast asleep and dreaming. What were they dreaming of? That was easy to guess. Blue-green water, clear as crystal, enchanting little islands, big white clouds flying across an enormous blue sky, and birds, birds, birds...travelling far, travelling far, travelling far.
I'm glad you've enjoyed rediscovering it too. I still love Susan's ingenious idea that enables the children to continue looking for the treasure without breaking their promise to their grandparents — and the race to find it before Mr "Pots of Money" does!!