I like the idea of penalties/bonuses depending on speed delivery. I think revenues should increase over time (inflation) but speed penalties should increas as well in a way that for an average performance compared to average transport speed (which also increase over time) should approximately be the same. If one is able to provide better speed he should be rewarded…
To be fair, in the game there already is a similar mechanism in that the higher the speed, the higher the goods/passengers can be delivered in the same time span which generates more revenues and thus profits. However, faster trains also cost a lot more in running fees… with supply not increasing as much (even considering that with added speed the 20 minutes limit means a larger radius) it often seems that is economically better to remain with older locomotives rather than invest in new ones… which does not economically makes sense.
So I think there should be some added incentives to renew one’s train fleet: speed bonuses, more supply, added city/industry growth