Dwell time is a crucial stake for railway operations corresponding to 20% of the total travel time in a mass transit context. A relevant margin must be allocated to the dwell time to guarantee the robustness of a timetable. Several studies have evaluated the robustness of the running time. However, except for a few heuristics, little work has tackled the issue of measuring the dwell time without margin named tight dwell time in this work. Then, we present two methods to estimate the tight dwell time from high resolution passenger flow data. Given the access to this data, both methods enable estimating the tight dwell time for all the stops while existing heuristics are limited to late trains and/or few passengers. Besides, our developed methods highlight the propensity of existing heuristics to overestimate what they measure. The estimation ex-post of the tight dwell time would help the design of future timetables.