No transportation company shall undertake the extension of its service or the construction of a new plant, property or facility, except ordinary extensions in the usual course of business, or shall acquire or operate any line, plant or property of another transportation company, unless and until there shall first have been obtained from the commission a certificate that the present or future public convenience and necessity require, or will require, or the public interest will be best served by, such extension, construction, acquirement, operation or acquirement and operation.