diff --git a/RELEASE-NOTES.md b/RELEASE-NOTES.md
index 951cf2497..d86a25713 100644
--- a/RELEASE-NOTES.md
+++ b/RELEASE-NOTES.md
@@ -19,6 +19,7 @@
 - **Divider** - Descenders like "g" are cut off in `horizontal divider` #3585
 - **Form Validation / Dropdown** - Using "enter" key in a `search dropdown` could cause a form to be submitted #3676
 - **Form Validation** - Revalidating a field `on: blur` could cause fields not yet interacted with to be validated #3606
+- **Rail** - Fixed incorrect width for `close rail` and `very close rail` caused by variable addition with mixed units `px` + `em` #3835
 - **Search** - A previous unfinished XHR query aborting would cause the next query to fail #2779
 
 **Bugs**
diff --git a/src/themes/default/elements/rail.variables b/src/themes/default/elements/rail.variables
index 969d91842..321a26d3d 100644
--- a/src/themes/default/elements/rail.variables
+++ b/src/themes/default/elements/rail.variables
@@ -23,8 +23,8 @@
 @splitCloseDistance: (@closeDistance / 2);
 @splitVeryCloseDistance: (@veryCloseDistance / 2);
 
-@closeWidth: @width + @splitCloseDistance;
-@veryCloseWidth: @width + @splitVeryCloseDistance;
+@closeWidth: ~"calc("@width~" + "@splitCloseDistance~")";
+@veryCloseWidth: ~"calc("@width~" + "@splitVeryCloseDistance~")";
 
 /* Dividing */
 @dividingBorder: 1px solid @borderColor;