inputs are never used for some track units. The marginal
price for these would therefore be infinite. An alternative
approach would be to calculate the average prices each year
and use these averages in the estimation. Our first priority in
the paper is, however, to estimate elasticities and derive
(marginal) costs for track use from these estimates.
Knowledge of marginal prices is of no relevance as long
as the marginal prices are the same for each track unit within
each year. With marginal prices that are constant for each
track unit within each year, Eq. (1) equals