src/Pure/Thy/rail.ML

     fun output_rule (name, rail) =
       let
         val (rail', y') = vertical_range rail 0;
         val out_name =
           (case name of
-            Antiquote.Text s => output_text false s
+            Antiquote.Text "" => ""
+          | Antiquote.Text s => output_text false s
           | Antiquote.Antiq a => output_antiq a);
       in
         "\\rail@begin{" ^ string_of_int y' ^ "}{" ^ out_name ^ "}\n" ^
         output "" rail' ^
         "\\rail@end\n"