diff --git a/OpenRA.Mods.Common/OpenRA.Mods.Common.csproj b/OpenRA.Mods.Common/OpenRA.Mods.Common.csproj
index 61f03891a0..1d672dff30 100644
--- a/OpenRA.Mods.Common/OpenRA.Mods.Common.csproj
+++ b/OpenRA.Mods.Common/OpenRA.Mods.Common.csproj
@@ -800,6 +800,7 @@
     <Compile Include="Traits\Render\WithChargeAnimation.cs" />
     <Compile Include="Traits\Render\WithChargeOverlay.cs" />
     <Compile Include="WebServices.cs" />
+    <Compile Include="Traits\Conditions\LineBuildSegmentExternalCondition.cs" />
   </ItemGroup>
   <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
   <Target Name="AfterBuild">
diff --git a/OpenRA.Mods.Common/Traits/Conditions/LineBuildSegmentExternalCondition.cs b/OpenRA.Mods.Common/Traits/Conditions/LineBuildSegmentExternalCondition.cs
new file mode 100644
index 0000000000..c78414ef0d
--- /dev/null
+++ b/OpenRA.Mods.Common/Traits/Conditions/LineBuildSegmentExternalCondition.cs
@@ -0,0 +1,88 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2017 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using System.Collections.Generic;
+using System.Linq;
+using OpenRA.Traits;
+
+namespace OpenRA.Mods.Common.Traits
+{
+	[Desc("Applies a condition to connected line build segments.")]
+	public class LineBuildSegmentExternalConditionInfo : ConditionalTraitInfo, Requires<LineBuildInfo>
+	{
+		[FieldLoader.Require]
+		[Desc("The condition to apply. Must be included in the target actor's ExternalConditions list.")]
+		public readonly string Condition = null;
+
+		public override object Create(ActorInitializer init) { return new LineBuildSegmentExternalCondition(init.Self, this); }
+	}
+
+	public class LineBuildSegmentExternalCondition : ConditionalTrait<LineBuildSegmentExternalConditionInfo>, INotifyLineBuildSegmentsChanged
+	{
+		readonly HashSet<Actor> segments = new HashSet<Actor>();
+		readonly Dictionary<Actor, int> tokens = new Dictionary<Actor, int>();
+
+		public LineBuildSegmentExternalCondition(Actor self, LineBuildSegmentExternalConditionInfo info)
+			: base(info) { }
+
+		void GrantCondition(Actor self, Actor segment)
+		{
+			if (tokens.ContainsKey(segment))
+				return;
+
+			var external = segment.TraitsImplementing<ExternalCondition>()
+				.FirstOrDefault(t => t.Info.Condition == Info.Condition && t.CanGrantCondition(segment, self));
+
+			if (external != null)
+				tokens[segment] = external.GrantCondition(segment, self);
+		}
+
+		void RevokeCondition(Actor self, Actor segment)
+		{
+			int token;
+			if (!tokens.TryGetValue(segment, out token))
+				return;
+
+			tokens.Remove(segment);
+			if (segment.Disposed)
+				return;
+
+			foreach (var external in segment.TraitsImplementing<ExternalCondition>())
+				external.TryRevokeCondition(segment, self, token);
+		}
+
+		void INotifyLineBuildSegmentsChanged.SegmentAdded(Actor self, Actor segment)
+		{
+			segments.Add(segment);
+			if (!IsTraitDisabled)
+				GrantCondition(self, segment);
+		}
+
+		void INotifyLineBuildSegmentsChanged.SegmentRemoved(Actor self, Actor segment)
+		{
+			if (!IsTraitDisabled)
+				RevokeCondition(self, segment);
+			segments.Remove(segment);
+		}
+
+		protected override void TraitEnabled(Actor self)
+		{
+			foreach (var s in segments)
+				GrantCondition(self, s);
+		}
+
+		protected override void TraitDisabled(Actor self)
+		{
+			foreach (var s in segments)
+				RevokeCondition(self, s);
+		}
+	}
+}