I really love the Ladybird books in the Ladybird Children's Classics series. Although the classic stories are vastly simplified, they still keep the essence of the original book while being retold in a way that makes them suitable for younger readers. The Railway Children by Edith Nesbit has been retold by Joan Collins for Ladybird and illustrated beautifully by Kathie Layfield. It's part of Series 740 and I'm hoping to be able to collect more of the titles in this series.
The story starts with the three children, Roberta, Peter and Phyllis, who live happily with their parents in London until one day their Father is mysteriously taken away (he is falsely imprisoned for spying) and the children have to move away with their Mother to a cottage in the country. They aren't happy there until they discover the railway, and among other adventures they befriend a man who takes the same train every day and eventually helps with their Father's release.