As reported on https://commons.wikimedia.org/wiki/Commons:Help_desk#can_someone_help_me_clearing_the_cache%3F that https://upload.wikimedia.org/wikipedia/commons/thumb/a/ac/Transit_timetable.svg/759px-Transit_timetable.svg.png is an old image that wasn't updated also the file got reuploaded three times (with minor changes and two different users). I tried yesterday twice ?action=purge, without success.
Between the three uploads I uploaded it to https://commons.wikimedia.org/wiki/File:Test.svg without any obvious problems (but here the default preview-size changed).
Side note: The original SVG failed to render on https://commons.wikimedia.org/w/index.php?title=Commons:Commons_SVG_Checker&withJS=MediaWiki:CommonsSvgChecker.js but the three reuploads were rendered by Commons:Commons_SVG_Checker
For new preview-sizes Wikimedia needs some rendering-time, for available preview-sizes it takes the carched one.
What happens if someone presses ?action=purge? Which preview-sizes were imediatly rendered? (Can that result in an overload?) What is the maximal processing time? Is there a CPU-Time-limit or a real-time-limit, because failures seems to be quite random after approxmiatly 2 real-time-seconds.
Is there a reduction of time-limit for high-traffic users? Because if I do many demanding requests get T226318 .