Those are both in "The Book of Railways" by Arthur Groom, as suggested by petethemole in his post of 1st March. I bought one dead cheap to see if that was it, and although it's similar and the girl in it is called Betty, it isn't the one I was thinking of. It's fascinating reading though - especially the 50s-style of speaking, which comes across as very stilted and old-fashioned. Mind you, I don't think anyone down our way spoke like BBC announcers in real life!
I have just recalled another quote from the book I am trying to trace - it ended with a mention of being able to travel to places by bus and a sentence something like "But who would go by bus when it was possible to travel by train? Certainly not [the children's names - which I remember as Dick and Betty Wilson]!"
I might well be confusing two or more books, and "The Book of Railways" may well have been one of them - after all it's 50-60 years ago...
Several people have mentioned 'The Book of Railways' by Arthur Groom, published by Birn Brothers Ltd., London, around 1950; featuring Betty and Billy Smith, with a series of full colour illustrations of the first years of British Railways by R.M. Clark and I gather that you now have a copy.
However, the lines, "Should we go by motor coach?" Mumsie said - followed by "How dreadful!" wailed the chorus of five (railway children), all together it seemed. "No, Mother. That can't be. Half the fun of the holiday would be gone. Let us stick to the railway," laughed Daddy. This text is found on page 47 of the little (6" x 4.5") book 'Off By Train', by George Gibbard Jackson, published, possibly 1928, by Raphael Tuck & Sons Ltd., London. The family is Mr. & Mrs. Burt and they have five children, Derek, Nell, Mary, John and Maureen!
The cover illustration, by Robert Barnard Way, is Royal Scot, No. 6117 'Welsh Guardsman (to traffic 10/1927) and the frontispiece, the only other colour picture, is of A1 Pacific, No. 4475 'Flying Fox (4/1923). All the other illustrations are in ink and unfortunately printed on poor quality paper.
In 1927, G.G. Jackson wrote 'The World's Railways', which was also illustrated by R.B. Way and I believe that 'Off By Train' uses some of these illustrations, but in a book aimed at younger children. Interestingly, the final chapter (from page 54), 'When the wind helped' has two ink drawings of engines with faces on their smoke-boxes, at least a decade before 'Sammy the Shunter' and almost two decades before Awdry's all-conquering 'Thomas'.
Hope this helps.
All the best,