It´s not that simple. RRT use a really simplified model where they partly just use unite load and don´t model hill correctly (and transport tycoon). In Locomotion they use a little more sophisticated model where they moddle weight of cargo in regard of acceleration, but still use a simplified models in hill-climbing (and decenting).
In train-fever the speed of the train is not modeled, it´s simulated. The game don´t know how fast the train is going to run trow the corner.
I guess the game uses a secondary algorithm to guesstimate the travletime, but its really hard to get it correct with out a pure simulation. One way of dealing with this could be to pause the game for a tenth of a second, run a instant simulation and show the result, or letting the game engine run a “dirty” simulation simultaneously and plot out the simulation on a graph. The graph i done over could be considers a simple dirty simulation. If i had a rail-line specification i could easily make a somewhat god approximation of the total travletime