diff --git a/tests/rails.scad b/tests/rails.scad
index a9fb0e9..7616169 100644
--- a/tests/rails.scad
+++ b/tests/rails.scad
@@ -23,6 +23,7 @@ use <../utils/layout.scad>
 use <../vitamins/nut.scad>
 
 sheet = 3;
+pos = 1; //[-1 : 0.1 : 1]
 
 module rails()
     layout([for(l = rails) carriage_width(rail_carriage(l))], 20)
@@ -33,7 +34,7 @@ module rails()
             nut = screw_nut(screw);
             washer = screw_washer(screw);
 
-            rail_assembly(rail, length, rail_travel(rail, length) / 2, $i<2 ? grey(20) : "green", $i<2 ? grey(20) : "red");
+            rail_assembly(rail, length, pos * rail_travel(rail, length) / 2, $i<2 ? grey(20) : "green", $i<2 ? grey(20) : "red");
 
             rail_screws(rail, length, sheet + nut_thickness(nut, true) + washer_thickness(washer));
 
diff --git a/vitamins/rail.scad b/vitamins/rail.scad
index 804f48e..f84914e 100644
--- a/vitamins/rail.scad
+++ b/vitamins/rail.scad
@@ -52,8 +52,9 @@ function rail_holes(type, length) = //! Number of holes in a rail given its ```l
 module rail_hole_positions(type, length, first = 0, screws = 100, both_ends = true) { //! Position children over screw holes
     pitch = rail_pitch(type);
     holes = rail_holes(type, length);
-    for(i = [first : holes - 1 - first])
-        if(i < screws || (holes - i <= screws && both_ends))
+    last = first + screws;
+    for(i = [first : holes - 1], j = holes - 1 - i)
+        if(i < last || both_ends && (j >= first && j < last))
             translate([i * pitch - length / 2 + (length - (holes - 1) * pitch) / 2, 0])
                 children();
 }
@@ -177,7 +178,6 @@ module rail_assembly(type, length, pos, carriage_end_colour = grey(20), carriage
 
     translate([pos, 0])
         carriage(rail_carriage(type), type,  carriage_end_colour, carriage_wiper_colour);
-
 }
 
 module rail_screws(type, length, thickness, screws = 100) { //! Place screws in the rail
@@ -193,6 +193,6 @@ module rail_screws(type, length, thickness, screws = 100) { //! Place screws in
             screw(end_screw, end_screw_len);
 
     translate_z(rail_screw_height(type, screw))
-        rail_hole_positions(type, length, index_screws, screws)
+        rail_hole_positions(type, length, index_screws, min(screws, rail_holes(type, length)) - 2 * index_screws)
             screw(screw, screw_len);
 }