And the Eastern Railroad Company shall locate and construct such tracks and bridge structures as shall be required to connect its present tracks northerly of its crossing with the Boston and Maine Railroad with the tracks and property so purchased or taken by it, keeping at all times east of a line drawn from a point on the easterly side of its present location, distant southerly three hundred and fifty feet, measured on said line from its intersection with the southerly side of Cambridge Street to the point of intersection of the northerly line of the state prison wharf with the easterly line of the location of the Boston and Maine Railroad, and thence keeping east of said easterly line of said location; and shall not thereafter cross the tracks of the Boston and Maine Railroad. And the Eastern Railroad Company shall take any lands now belonging to the Boston and Maine Railroad in Charlestown or Somerville lying easterly of such new location; and the Boston and Maine Railroad shall take all the road-bed, land and property of the Eastern Railroad Company lying between the line above described for the westerly limitation of said new location of the Eastern Railroad and the westerly line of the old location of the Eastern Railroad, and the present northerly line of the Fitchburg Railroad: _provided_, _however_, that in case of the aforesaid taking and exchange of property by and between the Eastern and Fitchburg Railroads, the Boston and Maine Railroad shall release the Eastern Railroad Company from all damages for its taking and occupation thereof and take from the said Eastern Railroad Company so much of the premises described in the first section of the three hundred and fifty-sixth chapter of the acts of the year eighteen hundred and seventy-two, as was taken from the said Boston and Maine Railroad by said Eastern Railroad Company under the provisions of that act; and said Eastern Railroad Company shall, without other compensation therefor, release to said Boston and Maine Railroad all their rights in said premises acquired by them, taking the same under said act; and _provided_, _further_, that any exchange of land made under the provisions of this section shall take effect simultaneously.