Sounds like it's from "The Park in the Dark", a beautifully illustrated children's book by Irish author Martin Waddell- it was a favourite of my childrens' (and mine).

And the story, briefly, from www.amazon.co.uk is ...

A hugely endearing picture book from the makers of the Big Bear and Little Bear series. When the sun goes down and the moon comes up, the toy monkey and his friends, Loopy the elephant and Little Gee the dog, creep softly out of the bedroom, down the stairs and through the dark hall on their way to the deserted park. The park's an exciting place to be after dark, though a little bit scary - especially when the Thing (a night train) hurtles out of the darkness, seeming to chase them. Then the three friends run all the way home and jump back into bed, safe and sound once more. Now a classic story, The Park in the Dark won the 1989 Kurt Maschler Award and has now been reissued to appeal to a new generation of young children.