correct power calculation

Author: peternewell

Source/Orts.Simulation/Simulation/RollingStocks/MSTSSteamLocomotive.cs

@@ -5838,6 +5838,15 @@ protected override void UpdateTractiveForce(float elapsedClockSeconds, float loc
             MaxPowerW = 0;
             MaxForceN = 0;
 
+            if (WheelSlip && AdvancedAdhesionModel)
+            {
+                AbsTractionSpeedMpS = AbsWheelSpeedMpS;
+            }
+            else
+            {
+                AbsTractionSpeedMpS = AbsSpeedMpS;
+            }
+
             // Update tractive effort across all steam engines
             for (int i = 0; i < SteamEngines.Count; i++)
             {