Nashville C. & St. L. Ry. v. Wallace, 288 U.S. 249 (1933).

Company sought declaratory judgment that tax was unconstitutional burden on interstate commerce
Matter was found to be justiciable: it would have been fine as an injunction request, so still fine as a declaratory judgment request—no need to follow exact procedures available in 1789.
Court focused on substance over form and emphasized that it was justiciable "so long as the case retains teh essentials of an adversary proceeding, involving a real, not a hypothetical, controversy."