Diesel engine common rail systems are expected to reach higher pressures, which require higher internal fatigue strength of the rail. The primary objective of this study is to improve the rail strength by modifying its structure. In this paper, a prospective rail construction capable of handling higher pressures is proposed. At first, a current generation rail was evaluated for pressure cycle durability, in order to analyze its strength. Then the optimized structure was sought via FEM analysis, which revealed that a flat internal surface significantly improved strength. The results were confirmed by pressure cycle tests.