The Allies recognised a simple, undeniable fact when planning Operation OVERLORD: Rommel must not be able to muster inland reinforcements to crush the invasion forces at the Normandy beachhead. The very success of D-Day depended on it. So began perhaps the most important phase of the OVERLORD air plan; the sustained and devastating assault of railways and bridges in northern Europe. The German occupiers were about to feel the full effect of the Allies' "Transportation Plan". Elliott Marsh writes for GAR.