This paper presents a new method to calculate the most suitable travel speed for high frequency railway lines to achieve as much capacity as possible for congested railway lines. The method calculates the most suitable travel speed based on the braking distance and information about the interlocking system. Based on the braking distance it is possible to calculate the minimum headway time, and thereby determine the buffer time when knowing the frequency. Hence the headway time can be divided into minimum headway time and buffer time. The buffer time is an indicator for the spare capacity of the railway line, and the more buffer time on the railway line, the better punctuality and the better possibilities to run more trains. Based on the described method a case example from the suburban railway lines of Copenhagen will be shown. The case example shows that a reduction of the maximum travel speed by 6% in central Copenhagen can increase the capacity by 11%. The increased capacity will improve the punctuality of the trains in central Copenhagen – even though some of the capacity will be used to run more trains through Copenhagen.