I have the feeling that the lightly written prose and formulaic story was put together rather quickly. The author's intuitions are basically sound.
My 5-year old, who loves cats and trains, is a committed fan and is still going strong after reading another two from the series. A book of this nature poses a problem in that it is too insubstantial for a good read aloud, but by the time the child is old enough to read paperbacks the subject matter will be too babyish.
Of the 3 I have read, the best was the railway cat's secret and the worst was the railway cat and the horse.