The outcome of a public railway investment appraisal depends on the assumed timetables with and without the investment. Yet principles for timetable design in appraisal are not explicitly discussed in the literature,neither in national appraisal guidelines nor in the research literature.The purpose of this paper is to explain and illustrate the crucial importance of timetable assumptions in public railway investment appraisal,why explicit principles for appraisal timetable construction are necessary,discuss the potential problems that the lack of such principles creates,discuss
different such principles,and explain why current practice will
tend to exaggerate investment benefits.