we used the ones that were there first (set at 3 initially and they went immediately down to 1 going over obstacles etc )
Coal loses only 1 pip for crossing any number of obstacles. It is NOT 1 pip per obstacle.
One needs to read the rule carefully (my emphasis):
If a barge moves over at least one obstacle space, the value of the transported coal die is decreased by 1
This will slow down the degradation of coal in your game I suspect.