This is the "empty element" of the Icons for railway descriptions (see there for further informations). The original syntax of Wikipedia:Route diagram templates filled all empty spaces in a diagram with this file (all BSicons were 20x20px). For those cells where an icon ID was provided, some "File:BSicon_ID.svg" were used, and where there was no ID, "File:BSicon_.svg" was substituted. Many major Wikipedias has since upgraded the syntax so that just an empty table cell is used (incl. En.WP).