The Supreme Court of Canada favours a flexible conception of federalism, in accordance with the principles of cooperative federalism, coordinated interpretation of powers and subsidiarity. It generally favours the simultaneous application of validly enacted legislation by Parliament and the provinces. There are cases where, however, the Court considers that the powers of one level of government must be protected against encroachment by the other. The doctrine of interjurisdictional immunity and federal paramountcy addresses this type of situation. The first makes it impossible to apply a provincial law that seriously impairs the essential content of a federal head of power. The second may render inoperative a provincial law that contradicts the letter or the spirit of a federal law. The Supreme Court has repeatedly pointed out that the doctrines of interjurisdictional immunity and federal paramountcy do not represent the mainstream of Canadian constitutional law, since they have the serious effect of rendering validly adopted provincial laws ineffective. The criteria for finding an impairment with the core of a federal head of power or an actual conflict of laws are therefore fairly strict. As we will see in this text, the recent jurisprudence of Canadian courts tends, on the contrary, to broaden these doctrines in favour of a more rigid conception of federalism favouring enclaves around federal head of powers over transportation, to the detriment of provincial and municipal jurisdictions on the environment and land use planning. While the Supreme Court condemned the asymmetrical application of these doctrines in the Canadian Western Bank case in 2007, some judges may already be returning to a more centralized approach as long as federal transportation are at issue in a litigation.