After the construction of a grade separation under this article, where the highway is carried over the railroad the state shall maintain the state highway and the structures supporting it and the drainage thereof, and the railroad company shall maintain its tracks; and where the state highway is passed under the railroad then the state shall maintain the highway and the drainage thereof, and the railroad company shall maintain its roadbed and the tracks and the structures supporting the same: Provided, however, That the state, at its sole expense, shall bear the cost of repairing or replacing any part of such supporting structure which may be damaged or destroyed by highway traffic; and the railroad company, at its own sole expense, shall bear the cost of repairing or replacing any part of such supporting structure which may be damaged or destroyed by railroad traffic: Provided further, That the provisions of this section shall not be applicable to grade separations constructed prior to the effective date of this act.