In looking at the resources for this game (read: wikias) I've noticed people making reference to an engine's WSP or Waggon-Speed Product. This metric is inherently flawed in that it does not take into account the time that an engine spends accelerating from it's standing start.
After crunching the numbers and running equations (yay math!) I've come up with a possible replacement for this metric. Although mine doesn't give a straight-up number it does give you a simple equation that can be used to compare two engines for running the same route. I present the Break-Even Equation:
or in game terms: MaxSpeed*(t-(MaxSpeed/2*Acceleration))
To use this to compare engines take their equations and set them equal to each other then solve for t. This will give you the time it will take for the slower accelerating train to overtake the faster accelerating train. If the t value is negative that means that one of the trains is both faster overall and faster accelerating. These equations are most useful to determine whether or not a faster acceleration is worth a tradeoff in speed for a given run.
This is just a basic equations and I have derived versions of this that take into account an engine's condition % and the number of waggons it hauls. These more complex equations, as well as the math I used to get the basic version can be supplied upon request.