The provisions of the laws of this commonwealth relative to the erection of bridges over rivers, creeks or rivulets, be and the same are hereby extended to the erection of bridges over canals and railroads: Provided, That bridges erected under the provisions of this act shall not obstruct any canal or railroad over which such bridge may be erected: And provided further, That nothing in this act shall release railroad, or other companies, or the commonwealth, from the requirements of existing laws.