All railroads and extensions of railroads hereafter constructed shall cross existing railroads and highways by passing either over or under the same, when practicable, and shall in no instance cross any railroad or highway at grade without authority first being obtained from the commission to do so. All highways and extensions of highways hereafter laid out and constructed shall cross existing railroads by passing either over or under the same, when practicable, and shall in no instance cross any railroad at grade without authority first being obtained from the commission to do so: PROVIDED, That this section shall not be construed to prohibit a railroad company from constructing tracks at grade across other tracks owned or operated by it within established yard limits. In determining whether a separation of grades is practicable, the commission shall take into consideration the amount and character of travel on the railroad and on the highway; the grade and alignment of the railroad and the highway; the cost of separating grades; the topography of the country, and all other circumstances and conditions naturally involved in such an inquiry.