Saw this while riding along a rail-trail route west of Philadelphia. Evidently they left the catenary support structures standing over the trail to carry the (still energized!) power lines, but the I-beams webs across the trail have rusted completely through.
Verily, rust never sleeps… I wonder if they ran diesel locomotives along this part of the line?
The beams across the active railway seem to be in fine shape.
The Smell of Molten Projects in the Morning 
I wonder if leakage current helped contribute to this some how?
The rust is most severe along the bottom half of each web, reducing it to a filigree. The next beam (lower in the picture) has the same see-through pattern. Notice that they’re rusted out near the corners of the support columns, too.
Highway overpasses show the same pattern of web corrosion over the travel lanes and I’ve always thought it was (high sulfur) diesel exhaust attacking the steel. But this is so uniform that maybe it’s just what happens after a century of exposure to the weather (and, I’d guess, pigeon crap).
I don’t know if that section of rail used AC or DC traction motors. Perhaps induced currents plus acidic dirt did the deed?
Every single beam along that entire stretch of trail showed the same pattern of missing metal. One of these days a great wind will topple a row of outer towers, folding those beams like cardboard where there’s no stiffening web. Best to not be riding the trail that day…
Interestingly, I just watched an episode of “World’s Toughest Fixes” where the host was helping replace insulators on Philly’s Septa system. The insulators they were removing were marked “1931”, so they’d been in service for a while. Hard to believe that such filthy porcelain has much insulating ability!
Given the tension cross-braces, I think the poles can withstand wind buffeting even once the crossbeams fall on the tracks. But I’d still be wary of riding the trail in bad weather!
insulators on Philly’s Septa system
This was out along the Schuylkill towards Pottstown, so those same insulators could be hanging off to the left over the mainline tracks… with the same grime holding them together!