diff --git a/vitamins/rail.scad b/vitamins/rail.scad
index b1d8ea6..88b946c 100644
--- a/vitamins/rail.scad
+++ b/vitamins/rail.scad
@@ -33,6 +33,9 @@ function rail_bore_depth(type) = type[7];   //! Counter bore depth
 function rail_screw(type)      = type[8];   //! Screw type
 function rail_carriage(type)   = type[9];   //! Carriage type
 function rail_end_screw(type)  = type[10];  //! Screw used for ends only (Countersink used for better location)
+function rail_groove_offset(type)=type[11]; //! Offset of centre of groove from top of rail
+function rail_groove_width(type)=type[12];  //! Groove width
+
 function rail_screw_height(type, screw) = rail_height(type) - rail_bore_depth(type) + screw_head_depth(screw, rail_hole(type)); //! Position screw taking into account countersink into counterbored hole
 function rail_travel(type, length) = length - carriage_length(rail_carriage(type)); //! How far the carriage can travel
 
@@ -134,43 +137,41 @@ module carriage(type, rail, end_colour = grey(20), wiper_colour = grey(20)) { //
             carriage_end(type, end_w, end_h, end_l);
 }
 
-module rail(type, length) { //! Draw the specified rail
+module rail(type, length, colour = grey(90), use_polycircles = false) { //! Draw the specified rail
     width = rail_width(type);
     height = rail_height(type);
 
     vitamin(str("rail(", type[0], ", ", length, "): Linear rail ", type[0], " x ", length, "mm"));
 
-    color(grey(90)) {
+    color(colour) {
         linear_extrude(height - rail_bore_depth(type)) difference() {
             square([length, width], center = true);
             rail_hole_positions(type, length)
-                 circle(d = rail_hole(type));
+                if (use_polycircles)
+                    poly_circle(r = rail_hole(type) / 2);
+                else
+                    circle(r = rail_hole(type) / 2);
         }
-
-        translate_z(rail_height(type) - rail_bore_depth(type)) {
-            h1 = rail_bore_depth(type) > 2 ? rail_bore_depth(type) / 3 : rail_bore_depth(type) / 2;
-            h0 = rail_bore_depth(type) > 2 ? (rail_bore_depth(type) - h1) / 2 : 0;
-            h2 = rail_bore_depth(type) - h1 - h0;
-
-            linear_extrude(h0) difference() {
-                square([length, width], center = true);
+        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);
                 rail_hole_positions(type, length)
-                     circle(d = rail_bore(type));
+                    if (use_polycircles)
+                        poly_circle(r = rail_bore(type) / 2);
+                    else
+                        circle(r = rail_bore(type) / 2);
             }
-            translate_z(h0)
-                linear_extrude(h1) difference() {
-                    w1 = max(width - 2, rail_bore(type));
-                    square([length, w1], center = true);
-                    rail_hole_positions(type, length)
-                        circle(d = rail_bore(type));
-                }
-            translate_z(h0 + h1)
-                linear_extrude(h2) difference() {
-                    square([length, width], center = true);
-                    rail_hole_positions(type, length)
-                        circle(d = rail_bore(type));
-                }
-        }
+
+        go = height - rail_groove_offset(type);
+        gw = rail_groove_width(type);
+        gd = gw / 2;
+        sw = (width - rail_bore(type)) / 2;
+        for (m = [0, 1])
+            mirror([0, m, 0])
+                translate([0, -width / 2])
+                    rotate([0, -90, 0])
+                        linear_extrude(length, center = true)
+                            polygon([ [0, 0], [0, sw], [height, sw], [height, 0], [go + gw/2, 0], [go, gd], [go - gw/2, 0] ]);
     }
 }
 
diff --git a/vitamins/rails.scad b/vitamins/rails.scad
index 0600a7b..02a6761 100644
--- a/vitamins/rails.scad
+++ b/vitamins/rails.scad
@@ -32,14 +32,14 @@ SSR15_carriage = [ 40.3, 23.3, 34, 24, 4.5, 0,  26, M4_cap_screw ];
 // Rails
 //
 //
-//                Wr  Hr   E    P   D    d    h
-MGN5 = [ "MGN5",  5,  3.6, 5,   15, 3.6, 2.4, 0.8, M2_cs_cap_screw, MGN5_carriage, M2_cs_cap_screw ]; // Screw holes too small for M2 heads
-MGN7 = [ "MGN7",  7,  5,   5,   15, 4.3, 2.4, 2.6, M2_cap_screw,    MGN7_carriage, M2_cs_cap_screw ];
-MGN9 = [ "MGN9",  9,  6,   7.5, 20, 6.0, 3.5, 3.5, M3_cap_screw,    MGN9_carriage, M3_cs_cap_screw ];
-MGN12= [ "MGN12", 12, 8,   10,  25, 6.0, 3.5, 4.5, M3_cap_screw,   MGN12_carriage, M3_cs_cap_screw ];
-MGN12H=[ "MGN12H",12, 8,   10,  25, 6.0, 3.5, 4.5, M3_cap_screw,   MGN12H_carriage,M3_cs_cap_screw ];
-MGN15= [ "MGN15", 15, 10,  10,  40, 6.0, 3.5, 5.0, M3_cap_screw,   MGN15_carriage, M3_cs_cap_screw ];
-SSR15= [ "SSR15", 15, 12.5,10,  60, 7.5, 4.5, 5.3, M4_cap_screw,   SSR15_carriage, M4_cs_cap_screw ];
+//                Wr  Hr   E    P   D    d    h                                                      go    gw
+MGN5 = [ "MGN5",  5,  3.6, 5,   15, 3.6, 2.4, 0.8, M2_cs_cap_screw, MGN5_carriage, M2_cs_cap_screw,  1,    1 ]; // Screw holes too small for M2 heads
+MGN7 = [ "MGN7",  7,  5,   5,   15, 4.3, 2.4, 2.6, M2_cap_screw,    MGN7_carriage, M2_cs_cap_screw,  1.5,  1.5 ];
+MGN9 = [ "MGN9",  9,  6,   7.5, 20, 6.0, 3.5, 3.5, M3_cap_screw,    MGN9_carriage, M3_cs_cap_screw,  1.5,  1.5 ];
+MGN12= [ "MGN12", 12, 8,   10,  25, 6.0, 3.5, 4.5, M3_cap_screw,   MGN12_carriage, M3_cs_cap_screw,  2.25, 2.75 ];
+MGN12H=[ "MGN12H",12, 8,   10,  25, 6.0, 3.5, 4.5, M3_cap_screw,   MGN12H_carriage,M3_cs_cap_screw,  2.25, 2.75];
+MGN15= [ "MGN15", 15, 10,  10,  40, 6.0, 3.5, 5.0, M3_cap_screw,   MGN15_carriage, M3_cs_cap_screw,  2.5,  2.75 ];
+SSR15= [ "SSR15", 15, 12.5,10,  60, 7.5, 4.5, 5.3, M4_cap_screw,   SSR15_carriage, M4_cs_cap_screw,  2.5,  2.75 ];
 
 rails = [MGN5, MGN7, MGN9, MGN12, MGN12H, MGN15, SSR15];