Section 69. A domestic company may purchase from a street railway company incorporated under the laws of another state so much of the railway, franchise and property of such other company as is located or used and exercised within this commonwealth when the railway of such other company connects with, intersects, or forms a continuous line with that of the purchasing company; provided, that the facilities for travel on the railway of each of said companies within this commonwealth shall not thereby be diminished, or the rates of fare increased; and provided, that no such purchase shall be valid or binding until the terms thereof have been approved by the department as required by section fifty-four of chapter one hundred and fifty-nine.