Automatic merge of T1.5.1-870-ge0bf062eb and 18 pull requests

- Pull request #570 at 3539862: Experimental glTF 2.0 support with PBR lighting
- Pull request #839 at d00beb9: First phase of https://blueprints.launchpad.net/or/+spec/additional-cruise-control-parameters
- Pull request #876 at f92de76: docs: add source for documents previously on website to source Documentation folder
- Pull request #882 at 9c456aa: Blueprint/train car operations UI window
- Pull request #885 at 8f94333: feat: Add notifications to Menu
- Pull request #886 at 6c0785b: Scene viewer extension to TrackViewer
- Pull request #892 at 1f5ba4c: Signal Function OPP_SIG_ID_TRAINPATH
- Pull request #896 at 5866028: First implementation of https://blueprints.launchpad.net/or/+spec/specific-sounds-for-ai-trains
- Pull request #897 at 42f1dd9: feat: Improved system information collection
- Pull request #903 at 9bead33: Downloading route content (Github, zip)
- Pull request #907 at 9b0b04f: Bug fix for https://bugs.launchpad.net/or/+bug/2047300 Dynamic tracks disappear after long tunnel
- Pull request #911 at 6834af0: docs: Add refactoring as a special type of PR
- Pull request #912 at 3112a51: New Triple Valve Features Vol. 2
- Pull request #914 at 8e50570: Adjustments to Duplex steam
- Pull request #915 at 6d911d7: Correct calculation error with curve friction
- Pull request #916 at 722037c: Distributed Power Air Brake Synchronization
- Pull request #917 at fde18cf: Lighting Configuration Enhancements
- Pull request #918 at f7ba507: Manual - News topics updated since 1.5.1

Source/Orts.Simulation/Simulation/RollingStocks/SubSystems/Brakes/MSTS/AirSinglePipe.cs

@@ -1471,9 +1471,9 @@ protected static void PropagateBrakeLinePressures(float elapsedClockSeconds, Tra
                         {
                             // Only sync application/release on DP units if both the lead unit AND the DP lead unit are set to synchronize
                             // Lead locomotive will always be allowed to apply/release
-                            bool syncApplication = loco == lead ? true : loco.DPSyncTrainApplication && lead.DPSyncTrainApplication;
-                            bool syncRelease = loco == lead ? true : loco.DPSyncTrainRelease && lead.DPSyncTrainRelease;
-                            bool syncEmergency = loco == lead ? true : loco.DPSyncEmergency && lead.DPSyncEmergency;
+                            bool syncApplication = loco == lead || loco.DPSyncTrainApplication && lead.DPSyncTrainApplication;
+                            bool syncRelease = loco == lead || loco.DPSyncTrainRelease && lead.DPSyncTrainRelease;
+                            bool syncEmergency = loco == lead || loco.DPSyncEmergency && lead.DPSyncEmergency;
 
                         tempBrakePipeFlow = 0.0f;
 
@@ -1699,7 +1699,7 @@ protected static void PropagateBrakeLinePressures(float elapsedClockSeconds, Tra
                 MSTSLocomotive leadLoco = train.DPLeadUnits[i] as MSTSLocomotive;
                 BrakeSystem locoBrakeSystem = train.DPLeadUnits[i].BrakeSystem;
 
-                bool syncIndependent = leadLoco == lead ? true : leadLoco.DPSyncIndependent && lead.DPSyncIndependent;
+                bool syncIndependent = lead != null && (leadLoco == lead || (leadLoco.DPSyncIndependent && lead.DPSyncIndependent));
 
                 // Set loco brake pressure on all units with brakes cut in
                 // Only set loco brake pressure on DP units if lead loco AND DP loco are equipped to synchronize braking