Add information to manual

Author: peternewell

Source/Documentation/Manual/physics.rst

@@ -3218,6 +3218,21 @@ the ``ORTSEmergencyBrakingDisablesWSP (1)`` parameter.
 
 When WSP is active the brake cylinder pressure reading will go yellow in the extended HuD on the BRAKE INFORMATION screen.
 
+SME (sometimes also called SEM) Brake System
+--------------------------------------------
+
+SME braking is a straight air-brake system having an automatic emergency feature by means of which the simplicity of the straight air brake 
+is retained for service operation, but it also has the additional protection afforded by the automatic application of the brake in 
+case of a break-in-two or the bursting of a hose. SME braking is typically used on short DMU rail sets. SME braking is a form of electro-pneumatic (EP)
+brake system, however EP and SME equipped cars cannot be mixed together in the same consist.
+
+To activate SME braking, set ``BrakeSystemType ( SME )``. 
+
+The following brake tokens can be used with it:
+``TrainBrakesControllerSMEOnlyStart``
+``TrainBrakesControllerSMEFullServiceStart``
+``TrainBrakesControllerSMEHoldStart``
+``TrainBrakesControllerSMEReleaseStart``
 
 Dynamically Evolving Tractive Force
 ===================================

Source/Orts.Simulation/Common/Scripting/BrakeController.cs

@@ -263,7 +263,7 @@ public enum ControllerState
         SMEOnly,            // TrainBrakesControllerSMEOnlyStart
         SMEFullServ,        // TrainBrakesControllerSMEFullServiceStart
         SMESelfLap,         // TrainBrakesControllerSMEHoldStart
-        SMEReleaseStart,    // TrainBrakesControllerSEMReleaseStart
+        SMEReleaseStart,    // TrainBrakesControllerSMEReleaseStart
     };
 
     public static class ControllerStateDictionary

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

@@ -23,6 +23,10 @@
 namespace Orts.Simulation.RollingStocks.SubSystems.Brakes.MSTS
 {
 
+    // Detailed description of the operation of a SME brake system can be found in:  "Air brakes, an up-to-date treatise on the Westinghouse air brake as designed for passenger and 
+    // freight service and for electric cars" by Ludy, Llewellyn V., 1875- [from old catalog]; American Technical Society
+    // https://archive.org/details/airbrakesuptodat00ludy/page/174/mode/2up?q=%22SME+brake%22
+    
     public class SMEBrakeSystem : AirTwinPipe
     {
         public SMEBrakeSystem(TrainCar car)