Section 183-a of the Tax Law imposes a tax surcharge on every transportation and transmission corporation and association subject to section 183 of the Tax Law for the privilege of exercising its corporate franchise, or of doing business, or of employing capital, or of owning or leasing property in the Metropolitan Commuter Transportation District in a corporate or organized capacity, or of maintaining an office in such district. For purposes of this Part, the term Metropolitan Commuter Transportation District (hereinafter MCTD) embraces the City of New York (Counties of Bronx, Kings, New York, Queens and Richmond) and the Counties of Dutchess, Nassau, Orange, Putnam, Rockland, Suffolk and Westchester. The tax surcharge imposed by section 183-a of the Tax Law is computed on that portion of the taxpayer's section 183 tax attributable to its business activities carried on within the MCTD as determined by the rules described in this Part.