The value of the taxable property of each railroad used for railroad purposes shall be ascertained without including:
(a) Any part of the cost of improvements, relocation, reconstruction, elimination or avoidance of highway grade crossings, including State highways, made pursuant to Title 48 of the Revised Statutes, or pursuant to the provision of an agreement for any of such purposes with a municipality made after April 27, 1931, but such exemptions shall apply only to main lines and branches existing on April 27, 1931;
(b) The cost essential to the improvement, relocation or reconstruction of existing grade separation structures made after January 1, 1960, pursuant to said Title 48 or pursuant to a said agreement;
(c) Any part of the cost of installing or replacing protective devices for the protection of the traveling public at grade crossings, such devices to be exempt from and after the assessing date of January 1, 1960.
The exemptions specified in subparagraphs (a) and (b) above shall not apply to any additions and betterments voluntarily made by a railroad in connection with grade crossing projects.