Allow to accelerate trains slowly towards red signals from stand.

Previously, it was impossible to accelerate a standing train once it was ~10m in front of a red signal.

advtrains_interlocking/lzb.lua

@@ -33,6 +33,7 @@ local ADD_SLOW   =  1
 local ADD_FAST   =  7
 local ZONE_ROLL  =  2
 local ZONE_HOLD  =  5 -- added on top of ZONE_ROLL
+local ZONE_VSLOW =  2 -- When speed is <2, still allow accelerating
 
 local SHUNT_SPEED_MAX = 4
 
@@ -175,6 +176,14 @@ local function apply_control(id, train)
 				--train.debug = train.debug .. "BRAKE!!!"
 				return
 			end
+			if v1==0 and v0<2 then
+				i = advtrains.path_get_index_by_offset(train, i, -ZONE_VSLOW)
+				if i <= train.index then
+					-- roll control
+					train.ctrl.lzb = 2
+					return
+				end
+			else
 				i = advtrains.path_get_index_by_offset(train, i, -ZONE_ROLL)
 				if i <= train.index then
 					-- roll control
@@ -189,6 +198,7 @@ local function apply_control(id, train)
 				end
 			end
 		end
+	end
 	train.ctrl.lzb = nil
 end