Probably the book you are thinking of would be a collected edition of the 'Little Golden Books' series of children's stories. It's unlikely you'd get an original copy of the compendium you remember, unless you hunted through book collectors' websites, and some second-hand dealer catalogues. However, there is a modern version of the collection - 'A Treasury Of Little Golden Books' which includes 'The Saggy-Baggy Elephant' and one called 'Tootle The Train' which may be the train story to which you refer.
Children's stories, especially classics like these are being re-printed and re-packaged so that children from the original era, who are parents now, can enjoy them with their own children. You should be able to obtain a copy of the re-printed anthology from your local bookshop, and you might be interested to know that 'The Saggy-Baggy Elephant' is popular enough to have made the transition into video, and computer formats. Media may change, but classic stories live on - thank heavens!