Explain the greyed-out ready box and count cars in the depot

A disabled checkbox shows no tooltip, so the settings override that greys
out "ready to go" is now said under it. The consist vehicle count counted
units, so a multi-unit was one vehicle; it now counts cars like the stats bar.

StarterNG.Tests/ConsistTailTests.cs

@@ -21,6 +21,17 @@ public class ConsistTailTests
         _consist = new Consist(_catalog, new VehicleInfo(_catalog, installation.Physics));
     }
 
+    [Fact]
+    public void The_vehicle_count_counts_cars_not_units()
+    {
+        Append("b111-01");
+        var pair = Append("b111-02");
+        pair.Cars.Add(pair.Cars[0].Clone());
+
+        Assert.Equal(2, _consist.Count);
+        Assert.Equal(3, _consist.VehicleCount);
+    }
+
     [Fact]
     public void A_new_last_car_is_left_uncoupled_at_the_back()
     {

StarterNG/Domain/Consist.cs

@@ -35,6 +35,9 @@ public sealed class Consist : IReadOnlyList<ConsistItem>
 
     public int Count => _items.Count;
 
+    /// <summary>Every vehicle, a multi-unit counted car by car - <see cref="Count"/> counts units.</summary>
+    public int VehicleCount => _items.Sum(i => i.Cars.Count);
+
     public ConsistItem this[int index]
     {
         get => _items[index];

StarterNG/Views/Depot/VehicleDetails.cs

@@ -568,7 +568,7 @@ public sealed class VehicleDetails
 
         var count = new TextBlock
         {
-            Text = $"{App.Loc["VehicleCount"]}: {_consist.Count}",
+            Text = $"{App.Loc["VehicleCount"]}: {_consist.VehicleCount}",
             FontSize = 12,
             Foreground = DimBrush,
             VerticalAlignment = VerticalAlignment.Center
@@ -593,9 +593,7 @@ public sealed class VehicleDetails
                 ? over == ConsistReadyOverride.AlwaysReady
                 : trainset?.ReadyToGo ?? false
         };
-        ToolTip.SetTip(ready, overridden
+        ToolTip.SetTip(ready, App.Loc["ConsistReadyDesc"]);
-            ? App.Loc["ConsistReadyOverridden"]
-            : App.Loc["ConsistReadyDesc"]);
 
         ready.IsCheckedChanged += (_, _) =>
         {
@@ -605,7 +603,12 @@ public sealed class VehicleDetails
             _redraw();
         };
 
+        // A disabled box shows no tooltip, so why it is greyed out is said beside it.
+        if (!overridden)
             return Row(content: ready);
+        var why = PanelNote(App.Loc["ConsistReadyOverridden"]);
+        why.Margin = new Thickness(0);
+        return Row(content: new StackPanel { Spacing = 2, Children = { ready, why } });
     }
 
     private Button LoadToolButton(string text, Action onClick, bool secondary = false)