diff --git a/src/autoreplace_gui.cpp b/src/autoreplace_gui.cpp
index 62e8b1e85e..b145e34696 100644
--- a/src/autoreplace_gui.cpp
+++ b/src/autoreplace_gui.cpp
@@ -34,8 +34,6 @@
 
 #include "safeguards.h"
 
-void DrawEngineList(VehicleType type, const Rect &r, const GUIEngineList &eng_list, const Scrollbar &sb, EngineID selected_id, bool show_count, GroupID selected_group);
-
 static bool EngineNumberSorter(const GUIEngineListItem &a, const GUIEngineListItem &b)
 {
 	return Engine::Get(a.engine_id)->list_position < Engine::Get(b.engine_id)->list_position;
@@ -114,27 +112,6 @@ class ReplaceVehicleWindow : public Window {
 		return true;
 	}
 
-	void AddChildren(const GUIEngineList &source, GUIEngineList &target, EngineID parent, int indent, int side)
-	{
-		for (const auto &item : source) {
-			if (item.variant_id != parent || item.engine_id == parent) continue;
-
-			const Engine *e = Engine::Get(item.engine_id);
-			EngineDisplayFlags flags = item.flags;
-			if (e->display_last_variant != INVALID_ENGINE) flags &= ~EngineDisplayFlags::Shaded;
-			target.emplace_back(e->display_last_variant == INVALID_ENGINE ? item.engine_id : e->display_last_variant, item.engine_id, flags, indent);
-
-			/* Add variants if not folded */
-			if ((item.flags & (EngineDisplayFlags::HasVariants | EngineDisplayFlags::IsFolded)) == EngineDisplayFlags::HasVariants) {
-				/* Add this engine again as a child */
-				if ((item.flags & EngineDisplayFlags::Shaded) == EngineDisplayFlags::None) {
-					target.emplace_back(item.engine_id, item.engine_id, EngineDisplayFlags::None, indent + 1);
-				}
-				AddChildren(source, target, item.engine_id, indent + 1, side);
-			}
-		}
-	}
-
 	/**
 	 * Generate an engines list
 	 * @param draw_left true if generating the left list, otherwise false
@@ -208,7 +185,7 @@ class ReplaceVehicleWindow : public Window {
 
 		this->engines[side].clear();
 		if (side == 1) {
-			AddChildren(list, this->engines[side], INVALID_ENGINE, 0, side);
+			GUIEngineListAddChildren(this->engines[side], list);
 		} else {
 			this->engines[side].swap(list);
 		}
diff --git a/src/build_vehicle_gui.cpp b/src/build_vehicle_gui.cpp
index db21d23c84..9d97603b3c 100644
--- a/src/build_vehicle_gui.cpp
+++ b/src/build_vehicle_gui.cpp
@@ -1045,6 +1045,9 @@ void DrawEngineList(VehicleType type, const Rect &r, const GUIEngineList &eng_li
 	int small_text_y_offset  = ir.Height() - GetCharacterHeight(FS_SMALL);
 	int replace_icon_y_offset = (ir.Height() - replace_icon.height) / 2;
 
+	const int offset = (rtl ? -circle_width : circle_width) / 2;
+	const int level_width = rtl ? -WidgetDimensions::scaled.hsep_indent : WidgetDimensions::scaled.hsep_indent;
+
 	int y = ir.top;
 	for (auto it = first; it != last; ++it) {
 		const auto &item = *it;
@@ -1052,6 +1055,21 @@ void DrawEngineList(VehicleType type, const Rect &r, const GUIEngineList &eng_li
 		bool has_variants = (item.flags & EngineDisplayFlags::HasVariants) != EngineDisplayFlags::None;
 		bool is_folded    = (item.flags & EngineDisplayFlags::IsFolded)    != EngineDisplayFlags::None;
 		bool shaded       = (item.flags & EngineDisplayFlags::Shaded)      != EngineDisplayFlags::None;
+
+		if (item.indent > 0) {
+			/* Draw tree continuation lines. */
+			int tx = (rtl ? ir.right : ir.left) + offset;
+			int ty = y - WidgetDimensions::scaled.matrix.top;
+			for (uint lvl = 1; lvl <= item.indent; ++lvl) {
+				if (HasBit(item.level_mask, lvl)) GfxDrawLine(tx, ty, tx, ty + step_size - 1, linecolour, WidgetDimensions::scaled.fullbevel.top);
+				if (lvl < item.indent) tx += level_width;
+			}
+			/* Draw our node in the tree. */
+			int ycentre = y + normal_text_y_offset + GetCharacterHeight(FS_NORMAL) / 2 - 1;
+			if (!HasBit(item.level_mask, item.indent)) GfxDrawLine(tx, ty, tx, ycentre, linecolour, WidgetDimensions::scaled.fullbevel.top);
+			GfxDrawLine(tx, ycentre, tx + offset - (rtl ? -1 : 1), ycentre, linecolour, WidgetDimensions::scaled.fullbevel.top);
+		}
+
 		/* Note: num_engines is only used in the autoreplace GUI, so it is correct to use _local_company here. */
 		const uint num_engines = GetGroupNumEngines(_local_company, selected_group, item.engine_id);
 
@@ -1079,14 +1097,6 @@ void DrawEngineList(VehicleType type, const Rect &r, const GUIEngineList &eng_li
 			Rect fr = ir.Indent(indent, rtl).WithWidth(circle_width, rtl);
 			DrawSpriteIgnorePadding(is_folded ? SPR_CIRCLE_FOLDED : SPR_CIRCLE_UNFOLDED, PAL_NONE, {fr.left, y, fr.right, y + ir.Height() - 1}, SA_CENTER);
 		}
-		if (indent > 0) {
-			/* Draw tree lines */
-			Rect fr = ir.Indent(indent - WidgetDimensions::scaled.hsep_indent, rtl).WithWidth(circle_width, rtl);
-			int ycenter = y + normal_text_y_offset + GetCharacterHeight(FS_NORMAL) / 2;
-			bool continues = std::next(it) != std::end(eng_list) && std::next(it)->indent == item.indent;
-			GfxDrawLine(fr.left + circle_width / 2, y - WidgetDimensions::scaled.matrix.top, fr.left + circle_width / 2, continues ? y - WidgetDimensions::scaled.matrix.top + step_size - 1 : ycenter, linecolour, WidgetDimensions::scaled.fullbevel.top);
-			GfxDrawLine(fr.left + circle_width / 2, ycenter, fr.right, ycenter, linecolour, WidgetDimensions::scaled.fullbevel.top);
-		}
 		y += step_size;
 	}
 }
@@ -1114,6 +1124,44 @@ void DisplayVehicleSortDropDown(Window *w, VehicleType vehicle_type, int selecte
 	ShowDropDownMenu(w, _engine_sort_listing[vehicle_type], selected, button, 0, hidden_mask);
 }
 
+/**
+ * Add children to GUI engine list to build a hierarchical tree.
+ * @param dst Destination list.
+ * @param src Source list.
+ * @param parent Current tree parent (set by self with recursion).
+ * @param indent Current tree indentation level (set by self with recursion).
+ */
+void GUIEngineListAddChildren(GUIEngineList &dst, const GUIEngineList &src, EngineID parent, uint8_t indent)
+{
+	for (const auto &item : src) {
+		if (item.variant_id != parent || item.engine_id == parent) continue;
+
+		const Engine *e = Engine::Get(item.engine_id);
+		EngineDisplayFlags flags = item.flags;
+		if (e->display_last_variant != INVALID_ENGINE) flags &= ~EngineDisplayFlags::Shaded;
+		dst.emplace_back(e->display_last_variant == INVALID_ENGINE ? item.engine_id : e->display_last_variant, item.engine_id, flags, indent);
+
+		/* Add variants if not folded */
+		if ((item.flags & (EngineDisplayFlags::HasVariants | EngineDisplayFlags::IsFolded)) == EngineDisplayFlags::HasVariants) {
+			/* Add this engine again as a child */
+			if ((item.flags & EngineDisplayFlags::Shaded) == EngineDisplayFlags::None) {
+				dst.emplace_back(item.engine_id, item.engine_id, EngineDisplayFlags::None, indent + 1);
+			}
+			GUIEngineListAddChildren(dst, src, item.engine_id, indent + 1);
+		}
+	}
+
+	if (indent > 0 || dst.empty()) return;
+
+	/* Hierarchy is complete, traverse in reverse to find where indentation levels continue. */
+	uint16_t level_mask = 0;
+	for (auto it = std::rbegin(dst); std::next(it) != std::rend(dst); ++it) {
+		auto next_it = std::next(it);
+		SB(level_mask, it->indent, 1, it->indent <= next_it->indent);
+		next_it->level_mask = level_mask;
+	}
+}
+
 /** Enum referring to the Hotkeys in the build vehicle window */
 enum BuildVehicleHotkeys {
 	BVHK_FOCUS_FILTER_BOX, ///< Focus the edit box for editing the filter string
@@ -1157,27 +1205,6 @@ struct BuildVehicleWindow : Window {
 		}
 	}
 
-	void AddChildren(const GUIEngineList &source, EngineID parent, int indent)
-	{
-		for (const auto &item : source) {
-			if (item.variant_id != parent || item.engine_id == parent) continue;
-
-			const Engine *e = Engine::Get(item.engine_id);
-			EngineDisplayFlags flags = item.flags;
-			if (e->display_last_variant != INVALID_ENGINE) flags &= ~EngineDisplayFlags::Shaded;
-			this->eng_list.emplace_back(e->display_last_variant == INVALID_ENGINE ? item.engine_id : e->display_last_variant, item.engine_id, flags, indent);
-
-			/* Add variants if not folded */
-			if ((item.flags & (EngineDisplayFlags::HasVariants | EngineDisplayFlags::IsFolded)) == EngineDisplayFlags::HasVariants) {
-				/* Add this engine again as a child */
-				if ((item.flags & EngineDisplayFlags::Shaded) == EngineDisplayFlags::None) {
-					this->eng_list.emplace_back(item.engine_id, item.engine_id, EngineDisplayFlags::None, indent + 1);
-				}
-				AddChildren(source, item.engine_id, indent + 1);
-			}
-		}
-	}
-
 	BuildVehicleWindow(WindowDesc *desc, TileIndex tile, VehicleType type) : Window(desc), vehicle_editbox(MAX_LENGTH_VEHICLE_NAME_CHARS * MAX_CHAR_LENGTH, MAX_LENGTH_VEHICLE_NAME_CHARS)
 	{
 		this->vehicle_type = type;
@@ -1518,7 +1545,7 @@ struct BuildVehicleWindow : Window {
 			default: NOT_REACHED();
 			case VEH_TRAIN:
 				this->GenerateBuildTrainList(list);
-				AddChildren(list, INVALID_ENGINE, 0);
+				GUIEngineListAddChildren(this->eng_list, list);
 				this->eng_list.shrink_to_fit();
 				this->eng_list.RebuildDone();
 				return;
@@ -1556,7 +1583,7 @@ struct BuildVehicleWindow : Window {
 		EngList_Sort(this->eng_list, _engine_sort_functions[this->vehicle_type][this->sort_criteria]);
 
 		this->eng_list.swap(list);
-		AddChildren(list, INVALID_ENGINE, 0);
+		GUIEngineListAddChildren(this->eng_list, list, INVALID_ENGINE, 0);
 		this->eng_list.shrink_to_fit();
 		this->eng_list.RebuildDone();
 	}
diff --git a/src/engine_gui.h b/src/engine_gui.h
index 5a43f1f610..ba7e41715f 100644
--- a/src/engine_gui.h
+++ b/src/engine_gui.h
@@ -11,6 +11,7 @@
 #define ENGINE_GUI_H
 
 #include "engine_type.h"
+#include "group_type.h"
 #include "sortlist_type.h"
 #include "gfx_type.h"
 #include "vehicle_type.h"
@@ -20,9 +21,10 @@ struct GUIEngineListItem {
 	EngineID engine_id;       ///< Engine to display in build purchase list
 	EngineID variant_id;      ///< Variant group of the engine.
 	EngineDisplayFlags flags; ///< Flags for toggling/drawing (un)folded status and controlling indentation.
-	int8_t indent;              ///< Display indentation level.
+	uint8_t indent; ///< Display indentation level.
+	uint16_t level_mask; ///< Mask of level continuations.
 
-	GUIEngineListItem(EngineID engine_id, EngineID variant_id, EngineDisplayFlags flags, int indent) : engine_id(engine_id), variant_id(variant_id), flags(flags), indent(indent) {}
+	GUIEngineListItem(EngineID engine_id, EngineID variant_id, EngineDisplayFlags flags, uint8_t indent) : engine_id(engine_id), variant_id(variant_id), flags(flags), indent(indent), level_mask(0) {}
 
 	/* Used when searching list only by engine_id. */
 	bool operator == (const EngineID &other) const { return this->engine_id == other; }
@@ -50,7 +52,10 @@ extern bool _engine_sort_show_hidden_engines[];
 extern const StringID _engine_sort_listing[][12];
 extern EngList_SortTypeFunction * const _engine_sort_functions[][11];
 
+/* Functions in build_vehicle_gui.cpp */
 uint GetEngineListHeight(VehicleType type);
 void DisplayVehicleSortDropDown(Window *w, VehicleType vehicle_type, int selected, WidgetID button);
+void DrawEngineList(VehicleType type, const Rect &r, const GUIEngineList &eng_list, const Scrollbar &sb, EngineID selected_id, bool show_count, GroupID selected_group);
+void GUIEngineListAddChildren(GUIEngineList &dst, const GUIEngineList &src, EngineID parent = INVALID_ENGINE, uint8_t indent = 0);
 
 #endif /* ENGINE_GUI_H */