The Pajares pass line is due to be replaced by a new base tunnel since many years now, but technical and financial difficulties have halted progress on the new line, and thus we can still enjoy heavy freight traffic on the 3.5% grades!
Perhaps the heaviest trains are steel coil trains, like this one, which are typically hauled by two class 253 TRAXX. But even these pairs are not able to pull the trains on the way up, thus the trains get split into two parts, one is hauled by the two 253s and the other one by a single japanese Bo-Bo-Bo electric (class 251). The two parts get united at Busdongo (first station after the summit tunnel), and continue their journey with just the two TRAXX, as pictured.
This was a very lucky shot, btw, because both 253s are pretty clean - usually even a single clean 253 is rare!