As written in the simutrans experimental thread, the current model in simutrans is correct for steam (there slightly wrong friction) engines, ships and airplanes (i.e. constant acceleration with quadratic drag). The only thing you add are acceleration for constant power (that is something that can be discussed) and several small stuff, which is not needed for the game world. Because I see no benefit in including friction factors for diferent waytypes. Especially since a sand road would have nearly a magnitude different one from a concrete highways.

One should keept in mind that there is not km/h in simutrans, at least not consistently. It is a game and using constant like g is completely useless (last time I checked the world was still flat in simutrans ...

)

About engines not reaching topspeed: WIth the old formular the topspeed was given by:

(total_power/total_weight+1)^0.5/2500

The new model it should be (if it is really like the wikipedia formula):

v=((sqrt(27*A*P^2+4*kr^3*m^3)+3^(3/2)*sqrt(A)*P)^(2/3)-2^(2/3)*kr*m)/(2^(1/3)*sqrt(3)*sqrt(A)*(sqrt(27*A*P^2+4*kr^3*m^3)+3^(3/2)*sqrt(A)*P)^(1/3))

with m: mass, kr:friction coefficent, A: air drag and P: power. (I love computer algebra systems ... ). These are two other solutions, but they are complex numberS (and thus make actually not much sense to me, since the speed should be a real number.)

So it can be done, but I am not sure, if this is really progress in the end.