The Translog cost function of Berndt and Christensen
(1972) is chosen as a flexible specification of the cost
structure Let i be an index for track unit, j an index for
district and t a time index. Cijt is the maintenance cost, Pkt
the marginal price for factor k; Yijt the track length, Uijt the
utilization level (gross weight) and zijt a vector of tracktechnical
variables (the number of switches, number of
tunnels, etc.). Then, the Translog specification is