Legally it is not a fine. It is a financial agreement between the two parties under which one party agrees not to take court action against the other - hence an “out of court settlement”.Not according to most English dictionaries.
By way of example
The basis of the settlement is the fares avoided (there is some debate as to the methodology used to calculate this) plus an amount to cover admin costs of the wronged party. Unlike a court case, there is no explicit punishment element to the settlement offer.