A quick look at Wikipedia for the PRR D5 indicates that the class comprised 18 engines (although there were more in different subclasses of these 4-4-0's).
Perhaps you were referring to the 4-4-0 "American" type, in general. I believe the "American" type was the most common locomotive of the 19th Century in the United States, and perhaps
the most numerous ever (but not sure).
