Index: trunk/src/org/openstreetmap/josm/actions/FollowLineAction.java
===================================================================
--- trunk/src/org/openstreetmap/josm/actions/FollowLineAction.java	(revision 4086)
+++ trunk/src/org/openstreetmap/josm/actions/FollowLineAction.java	(revision 4086)
@@ -0,0 +1,122 @@
+// License: GPL. For details, see LICENSE file.
+package org.openstreetmap.josm.actions;
+
+import static org.openstreetmap.josm.tools.I18n.tr;
+
+import java.awt.event.ActionEvent;
+import java.awt.event.KeyEvent;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.List;
+
+import org.openstreetmap.josm.Main;
+import org.openstreetmap.josm.actions.mapmode.DrawAction;
+import org.openstreetmap.josm.data.osm.Node;
+import org.openstreetmap.josm.data.osm.OsmPrimitive;
+import org.openstreetmap.josm.data.osm.OsmPrimitiveType;
+import org.openstreetmap.josm.data.osm.Way;
+import org.openstreetmap.josm.gui.layer.OsmDataLayer;
+import org.openstreetmap.josm.tools.Shortcut;
+
+/**
+ * Follow line action - Makes easier to draw a line that shares points with another line
+ *
+ * Aimed at those who want to draw two or more lines related with
+ * each other, but carry different information (i.e. a river acts as boundary at
+ * some part of its course. It preferable to have a separated boundary line than to
+ * mix totally different kind of features in one single way).
+ *
+ * @author Germán Márquez Mejía
+ */
+public class FollowLineAction extends JosmAction {
+
+    public FollowLineAction() {
+        super(
+                tr("Follow line"),
+                "followline.png",
+                tr("Continues drawing a line that shares nodes with another line."),
+                Shortcut.registerShortcut("tools:followline", tr(
+                "Tool: {0}", tr("Follow")),
+                KeyEvent.VK_F, Shortcut.GROUP_EDIT), true);
+    }
+
+    @Override
+    protected void updateEnabledState() {
+        if (getCurrentDataSet() == null) {
+            setEnabled(false);
+        } else {
+            updateEnabledState(getCurrentDataSet().getSelected());
+        }
+    }
+
+    @Override
+    protected void updateEnabledState(Collection<? extends OsmPrimitive> selection) {
+        setEnabled(selection != null && !selection.isEmpty());
+    }
+
+    @Override
+    public void actionPerformed(ActionEvent evt) {
+        OsmDataLayer osmLayer = Main.main.getEditLayer();
+        if (osmLayer == null)
+            return;
+        if (!(Main.map.mapMode instanceof DrawAction)) return; // We are not on draw mode
+        
+        Collection<Node> selectedPoints = osmLayer.data.getSelectedNodes();
+        Collection<Way> selectedLines = osmLayer.data.getSelectedWays();
+        if ((selectedPoints.size() > 1) || (selectedLines.size() != 1)) // Unsuitable selection
+            return;
+        
+        Node last = ((DrawAction) Main.map.mapMode).getCurrentBaseNode();
+        Way follower = selectedLines.iterator().next();
+        Node prev = follower.getNode(1);
+        boolean reversed = true;
+        if (follower.lastNode().equals(last)) {
+            prev = follower.getNode(follower.getNodesCount() - 2);
+            reversed = false;
+        }
+        List<OsmPrimitive> referrers = last.getReferrers();
+        if (referrers.size() < 2) return; // There's nothing to follow
+        
+        Iterator<OsmPrimitive> i = referrers.iterator();
+        while (i.hasNext()) {
+            OsmPrimitive referrer = i.next();
+            if (!referrer.getType().equals(OsmPrimitiveType.WAY)) { // Can't follow points or relations
+                continue;
+            }
+            Way toFollow = (Way) referrer;
+            if (toFollow.equals(follower)) {
+                continue;
+            }
+            List<Node> points = toFollow.getNodes();
+            int indexOfLast = points.indexOf(last);
+            int indexOfPrev = points.indexOf(prev);
+            if ((indexOfLast == -1) || (indexOfPrev == -1)) { // Both points must belong to both lines
+                continue;
+            }
+            if (Math.abs(indexOfPrev - indexOfLast) != 1) {  // ...and be consecutive
+                continue;
+            }
+            Node newPoint = null;
+            if (indexOfPrev == (indexOfLast - 1)) {
+                if ((indexOfLast + 1) < points.size()) {
+                    newPoint = points.get(indexOfLast + 1);
+                }
+            } else {
+                if ((indexOfLast - 1) >= 0) {
+                    newPoint = points.get(indexOfLast - 1);
+                }
+            }
+            if (newPoint != null) {
+                if (reversed) {
+                    follower.addNode(0, newPoint);
+                } else {
+                    follower.addNode(newPoint);
+                }
+                osmLayer.data.clearSelection();
+                osmLayer.data.addSelected(follower);
+                osmLayer.data.addSelected(newPoint);
+                return;
+            }
+        }
+    }
+}
Index: trunk/src/org/openstreetmap/josm/actions/mapmode/DrawAction.java
===================================================================
--- trunk/src/org/openstreetmap/josm/actions/mapmode/DrawAction.java	(revision 4085)
+++ trunk/src/org/openstreetmap/josm/actions/mapmode/DrawAction.java	(revision 4086)
@@ -752,4 +752,8 @@
     }
 
+    public Node getCurrentBaseNode() {
+        return currentBaseNode;
+    }
+
     private static void pruneSuccsAndReverse(List<Integer> is) {
         //if (is.size() < 2) return;
Index: trunk/src/org/openstreetmap/josm/gui/MainMenu.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/MainMenu.java	(revision 4085)
+++ trunk/src/org/openstreetmap/josm/gui/MainMenu.java	(revision 4086)
@@ -33,4 +33,5 @@
 import org.openstreetmap.josm.actions.DuplicateAction;
 import org.openstreetmap.josm.actions.ExitAction;
+import org.openstreetmap.josm.actions.FollowLineAction;
 import org.openstreetmap.josm.actions.FullscreenToggleAction;
 import org.openstreetmap.josm.actions.GpxExportAction;
@@ -161,4 +162,5 @@
     public final JosmAction joinAreas = new JoinAreasAction();
     public final JosmAction createMultipolygon = new CreateMultipolygonAction();
+    public final JosmAction followLine = new FollowLineAction();
 
     /* Audio menu */
@@ -330,4 +332,5 @@
         add(toolsMenu, mirror);
         toolsMenu.addSeparator();
+        add(toolsMenu, followLine);
         add(toolsMenu, addnode);
         add(toolsMenu, movenode);