As the examples show, the parts of a compound noun may be joined (with or without a hyphen) or written separately. In some words usage is fixed, in others it varies. The hyphen, however, is less used than formerly.
Note : The first part of a compound noun usually limits the second after the manner of an adjective. Indeed, many expressions may be regarded either (1) as compounds or (2) as phrases containing an adjective and a noun. Thus railway conductor may be taken as a compound noun, or as a noun (conductor) limited by an adjective (railway).