Whenever a common carrier by rail organized under the laws of this state shall issue (1) additional shares of its capital stock having unconditionally the general right to vote or (2) securities convertible into shares of such capital stock, it shall, except as hereinafter provided, first offer such shares or convertible securities proportionately to holders of its stock having such voting right in such manner as shall have been determined by its directors; provided, however, that if the charter, articles of agreement or stockholder votes authorizing the outstanding stock have limited or eliminated the rights of holders of such outstanding stock to subscribe to such shares or convertible securities, such offer need not be made, except to the extent, if any, required by said charter, articles of agreement or votes.