Accessing the original file returns a 404 error.

I know. I meant why it return 404.

Well, it looks gone. The file was uploaded in 2012, apparently still existed around a year ago: http://wayback.archive.org/web/20151001170542/https://es.wikipedia.org/wiki/Línea_7_(Metro_de_Madrid)#Recorrido

I guess you could reupload the 800px thumbnail, copied from the Wayback Machine page:

The files of course shouldn't be disappearing, but this isn't really my area of experience. Perhaps @aaron or @fgiunchedi can help. But from what I've seen of previous similar cases (this occurs regularly), it's probably gone for good.

Yay for mirrors! http://ftpmirror.your.org/pub/wikimedia/images/wikipedia/commons/c/cf/Metro_Mad_Linea_7.png

As far as I can see, nothing happened to the file since 2012. Files occasionally get lost on rename or delete, and that is bad, but losing them with no apparent reason is even more scary.

Apparently if the file had been uploaded a year later, we would be out of luck: T53001: Image tarball dumps on your.org are not being generated

Filed T153565: MediaWiki file operations are fragile, causing occasional data loss about what I think is the more fundamental issue (apart from the lack of reliable mirrors).

In T153540#2884321, @Tgr wrote:

Yay for mirrors! http://ftpmirror.your.org/pub/wikimedia/images/wikipedia/commons/c/cf/Metro_Mad_Linea_7.png

Uploaded; thanks for finding out the mirror. I guess this task can be resolved now? Further work would be in T53001 & T153565.