An alternative would be for somewhere in the locomotive-info screen, there would be a graph showing how the acceleration decreases as the mass increases, and maybe an additional graph showing how acceleration etc. is affected by slope. In Railroad Tycoon 2, I remember there were such graphs in the locomotive-info screen.
But having an “average speed in the past year” would be effective to see how the train has been handling on your actual track rather than on a hypothetical flat curveless obstructionless track.