diff --git a/railties/lib/rails/test_unit/line_filtering.rb b/railties/lib/rails/test_unit/line_filtering.rb
index 65f76f21a6e11201eed144d43e800e5dd9ba50a3..fb5ff231e4d8fbec06238a7cefffeb766b106636 100644
--- a/railties/lib/rails/test_unit/line_filtering.rb
+++ b/railties/lib/rails/test_unit/line_filtering.rb
@@ -18,12 +18,14 @@ def initialize(runnable, filter, patterns)
       @filters = [ derive_regexp(filter), *derive_line_filters(patterns) ].compact
     end
 
+    # Minitest uses === to find matching filters.
     def ===(method)
       @filters.any? { |filter| filter === method }
     end
 
     private
       def derive_regexp(filter)
+        # Regexp filtering copied from Minitest.
         filter =~ %r%/(.*)/% ? Regexp.new($1) : filter
       end