Automatic merge of T1.5.1-794-g4cad06780 and 14 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 a055bca: Blueprint/train car operations UI window
- Pull request #885 at 06b8a9e: 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 0a9d939: feat: Improved system information collection
- Pull request #899 at 776afa6: Duplex steam engines - Booster Engine addition
- Pull request #903 at 7353625: Downloading route content (Github, zip)
- Pull request #906 at 5850660: Bug fix for https://bugs.launchpad.net/or/+bug/2047299 Crash loading a 3Dcab-only loco
- Pull request #907 at 9b0b04f: Bug fix for https://bugs.launchpad.net/or/+bug/2047300 Dynamic tracks disappear after long tunnel
- Pull request #908 at 4b4afe3: feat: supports switching adhesion precisions

Source/Documentation/Manual/physics.rst

@@ -237,6 +237,16 @@ advanced adhesion model dynamics. The value considers the inertia of all
 the axles and traction drives. If not set, the value is estimated from the
 locomotive mass and maximal power.
 
+By inserting multiple "Axle" sections in the above configuration, multiple 
+indpependent wheelsets can be defined which will operate independently of 
+each other. The following parameters can be inserted to characterise the 
+performance of the wheelset.
+
+``AnimatedParts`` - animated parts associated with the axles wheelset.
+``Weight`` - weight on the axles in the wheelset.
+``ORTSRadius`` - radius of the wheels in the wheelset.
+``NumberWheelAxles`` - number of axles in the wheelset.
+
 The first model -- simple adhesion model -- is a simple tractive force
 condition-based computation. If the tractive force reaches its actual
 maximum, the wheel slip is indicated in HUD view and the tractive force
@@ -1253,6 +1263,42 @@ cylinder also tended to reach finite limits as well. These factors
 typically combined to place limits on the power of a locomotive depending
 upon the design factors used.
 
+Steam Locomotives with Multiple Engines
+.......................................
+
+Some steam locomotives can have multiple steam engines (ie separate steam 
+cylinders connected to different wheels), such as the 4-4-4-4 locomotive or 
+an articulated Garrat locomotive.
+
+To configure these types of locomotives configurations for multiple steam 
+engines need to be added to the engine section of the ENG file, would have the 
+following format::
+
+    ORTSSteamEngines ( x
+        Wheelset (
+           
+        )
+    )
+
+where x = number of steam engines fitted to locomotive.
+
+The following parameters can be used to configure the steam engine::
+
+``NumCylinders`` - number of steam cylinders in engine.
+``CylinderStroke`` - stroke of steam cylinder.
+``CylinderDiameter`` - diameter of steam cylinder.
+``MaxIndicatedHorsepower`` - maximum indicated horsepower of steam engine.
+``AttachedAxle`` - the axle wheelset that the steam engine is attached to.
+
+To specify the engine as a Booster engine the following additional parameters 
+can be used::
+
+``BoosterCutoff`` - the cutoff point for the Booster steam cylinder.
+``BoosterThrottleCutoff`` - the locomotive cutoff point where the Booster unlatches.
+``BoosterGearRatio`` - the gear ratio of the Booster engine.
+``AuxiliarySteamEngineType`` - by inserting "Booster" into this parameter the 
+engine is defined as a Booster engine.
+
 Locomotive Types
 ................