This is a draft text for a chapter in Handbook on Research Methods in Transport Economics and Policy to be published by Edward Elgar Publishing. It provides an overview of issues related to scarcity in scheduled transport modes with emphasis on railways. The paper separates the scarcity or time-tabling problem into two analytical parts. The first concerns the challenges related with finding an approximate solution to the mathematically challenging NP complete problem. The second generic problem is related to base the solution to this challenge on the operators’ value of each departure slot.