@Azrael: what you say is true, but you can replace old trains in service with a new locomotive of the same “old model”. Increase in speed and power is capable of offsetting higher running costs only in the early-mid part of the game. I am not sure if the added range of the 20 minutes limit with a higher speed is capable of offsetting added running costs. In my experience I tend to have more profitable lines, also for passengers, by choosing the locomotive with the lower running costs even if it has lower power and speed (except for the first part of the game), which is often an older model. Of course this could be balanced in other ways such as increasing running costs of older models with time due to inflation (but prices should also adjust), difficulties of finding spare parts etc.
@isidoro: it is true that those bonuses are somewhat unrealistic, but in some countries we have price discriminations for high speed passenger trains, faster mail delivery etc. I don’t know for freight. However, this is a game and it should be fun: many other transport games have this kind of mechanism and the game was funnier (of course they also had competition with other companies or players…).