Thanks "justme69" for your post above.
Looking at the aerial photograph of Barajas on
www.elpais.com, the latter portion of the runway does appear to be of a different material/colour. Am I right in assuming that aircraft are not expected routinely to use it? If so, this might explain the report that the Spanair aircraft used "all" the runway.
Apropos earlier comments about emergency services' access to the ravine, I was in Madrid on Wednesday and live coverage on television showed emergency vehicles descending into the ravine. Couldn't say how easy it was for them to do so, though.