current vehicle price model:
* scale €/km (or tile) linear with capacity and speed
* scale €/m (per month) nonlinear with speed
* €/m is a lot higher than €/km*max_driven_km
-yealds>
you have to drive at speedlimit or else you make a loss.

suggested vehicle price model:
* scale €/km nonlinear with speed
* scale €/m linear with speed and cap
* modern or aerodynamic vehicle should consume less €/km:
** (e.g. 50l/100km a 1970 40t 300HP Semi-Truck vs today 25-30l/100km 450HP
eg2. MB Lo 2000 4t 40 kW 16l/100km)
** steam high €/km (+10%) at same speed, diesel medium €/km (+-0%) at same speed, electric low €/km (-10%)
** aerodyamic locos or EMU (-10%)
-yealds>
you can drive as fast as possible but only if it's worth (speedbonus). You don't lose money by using modern (high cap, but high cost) waggons with slow speed loco and track (€/km*driven_km is much higher on speedlimit than €/m).
It's more like the real word physics for double the speed you need ^2 more power (fuel per 100km per kW or tonnage).