Infrastructure69G D A B C E HFig. 2.Sketch of a Railway Network and its Track Units.Except for spending on maintenance and traf volumes, Banverket registers the number of switches, bridges, tunnels and other technical installations on each separate track unit. In addition, a track quality index comprising eight classes, each representing the (average) standard level of sleepers, fastenings and rails is assigned to each track unit. For instance, index no. 8 �the best tracks �is given to track sections with the predominantly heaviest (per metre) whole welded rails on concrete sleepers. An administrative distinction is also made between track units that are part of a main or secondary line and those that are not. 20 maintenance districts �each responsible for a set of track units corresponding to a certain number of track km �are responsible for recording the data. The dataset comprises the 1994�996 period. While common costs could in principle be distributed over the track units using different keys, the analysis is only applied to track-speci costs. The presumption is that the agency common costs for main, regional or district ofes do not vary with the traf load; common costs affect average but not marginal, costs. Thus, the records account for about 1.6 out of the total spending on maintenance of 2.3 billion SEK in 1994. One implication is of course that this per se means that 30% of total costs are not to be made part of a marginal cost-charging tariff. Traf is measured as gross tonne km using each track unit, i.e. the number of km that trains of a certain weight �also including the weight of loco and cars �have traversed on each track unit. There are 169, 176 and 175 observations of costs per track unit available for the years 1994, 1995 and 1996. The translog cost function of Berndt and Christensen (1972), presented in Section 2.1 above, was used as a xible speciation of the cost structure. The al model used for the analysis is given by ln C ijt a at aj b yijt b uijt byy y2 buu u2 byu yijt uijt y u ijt ijt bI I ijt b0z zijt t 3:15
