diff --git a/libtest.png b/libtest.png
index 04b1238..3763fe3 100644
Binary files a/libtest.png and b/libtest.png differ
diff --git a/readme.md b/readme.md
index 7326623..3040856 100644
--- a/readme.md
+++ b/readme.md
@@ -2658,6 +2658,8 @@ Linear rails with carriages.
 | `rail_carriage(type)` | Carriage type |
 | `rail_end(type)` | Minimum distance screw can be from the end |
 | `rail_end_screw(type)` | Screw used for ends only (Countersink used for better location) |
+| `rail_groove_offset(type)` | Offset of centre of groove from top of rail |
+| `rail_groove_width(type)` | Groove width |
 | `rail_height(type)` | Height of rail section |
 | `rail_hole(type)` | Screw hole diameter |
 | `rail_pitch(type)` | Distance between screws |
@@ -2678,7 +2680,7 @@ Linear rails with carriages.
 |:--- |:--- |
 | `carriage(type, rail, end_colour = grey(20)` | Draw the specified carriage |
 | `carriage_hole_positions(type)` | Position children over screw holes |
-| `rail(type, length)` | Draw the specified rail |
+| `rail(type, length, colour = grey(90)` | Draw the specified rail |
 | `rail_assembly(type, length, pos, carriage_end_colour = grey(20)` | Rail and carriage assembly |
 | `rail_hole_positions(type, length, first = 0, screws = 100, both_ends = true)` | Position children over screw holes |
 | `rail_screws(type, length, thickness, screws = 100, index_screws = undef)` | Place screws in the rail |
diff --git a/tests/png/rails.png b/tests/png/rails.png
index bc29aa4..9eb9ae2 100644
Binary files a/tests/png/rails.png and b/tests/png/rails.png differ
diff --git a/vitamins/rail.scad b/vitamins/rail.scad
index 88b946c..24c791e 100644
--- a/vitamins/rail.scad
+++ b/vitamins/rail.scad
@@ -144,28 +144,32 @@ module rail(type, length, colour = grey(90), use_polycircles = false) { //! Draw
     vitamin(str("rail(", type[0], ", ", length, "): Linear rail ", type[0], " x ", length, "mm"));
 
     color(colour) {
+        rbr = rail_bore(type) / 2;
+        w = corrected_radius(rbr) * 2 + 2 * eps; // width of core big enough for the holes
         linear_extrude(height - rail_bore_depth(type)) difference() {
-            square([length, width], center = true);
+            square([length, w], center = true);
+
             rail_hole_positions(type, length)
                 if (use_polycircles)
-                    poly_circle(r = rail_hole(type) / 2);
+                    poly_circle(rail_hole(type) / 2);
                 else
-                    circle(r = rail_hole(type) / 2);
+                    circle(d = rail_hole(type));
         }
         translate_z(rail_height(type) - rail_bore_depth(type))
             linear_extrude(rail_bore_depth(type)) difference() {
-                square([length, rail_bore(type) + 2 * eps], center = true);
+                square([length, w], center = true);
+
                 rail_hole_positions(type, length)
                     if (use_polycircles)
-                        poly_circle(r = rail_bore(type) / 2);
+                        poly_circle(rbr);
                     else
-                        circle(r = rail_bore(type) / 2);
+                        circle(rbr);
             }
 
         go = height - rail_groove_offset(type);
         gw = rail_groove_width(type);
         gd = gw / 2;
-        sw = (width - rail_bore(type)) / 2;
+        sw = (width - w) / 2;
         for (m = [0, 1])
             mirror([0, m, 0])
                 translate([0, -width / 2])