Any railroad company heretofore incorporated, or which may be hereafter organized or incorporated in this state in pursuance of law, may receive subscriptions to the capital stock of such company or corporation to aid in the construction or equipment of its road, to be known as “transportation subscriptions”, and the certificates of stock to be issued thereon, as herein provided, shall be known as “transportation certificates”, each of which shall be for an amount equal to one share of stock in such company and may be issued to one or more persons, and shall entitle the holder or holders thereof, or their assigns or legal representatives, to all the privileges of a stockholder in such company, but shall not subject the holder to any liability for the debts or obligations of such company, and shall be an irrevocable and indefeasible first lien and charge upon and against such railroad, and the roadbed, rolling stock and depots, engine houses and machine shops of such company, then in possession of or thereafter acquired by such company or its successor or successors or assigns, whether in the hands of such company or of any other person, company or corporation whatsoever into whose hands or possession the same may at any time come, until taken up, paid or discharged by such company as herein provided, except the liens provided by law in favor of the employees of a railroad company, and in favor of parties performing labor, or furnishing materials for the construction or the improvement of the roadbed, rolling stock, station houses, depots, bridges or culverts, and except as to mortgages recorded in the county or counties through which the line of such road runs or is proposed to be run, before the date of the making of such subscription; which mortgages shall have the preference only as to so much of such road and the property of such company as lies or is situated in counties in which the same are at that time recorded; and such mortgage shall not have any preference as a lien upon any property of such company acquired after the time of making such subscriptions.