tunnel fur
n. A mixture of dust, oil, human hair and skin, rodent feces, and other substances that collects on subway tracks.
The air-conditioning is failing partly because of the age of the units, which both the workers’ union and transit agency agree should have been overhauled sooner. Their fragility has become clear this summer, with the heat forcing them to work harder even as a particularly foul mixture of substances known as “tunnel fur” has been jamming up the condensers.
—Oliver Moore, “John Tory rides sweltering subway, tells TTC to 'do better',” The Globe and Mail, September 07, 2016
AC compressors can become clogged with tunnel fur (debris like dust and grime) causing AC failures.
—Brad Ross, “AC compressors can…,” Twitter, September 07, 2016
I believe that's what the TTC refers to as "tunnel fur"
—ruckusss, “Does anyone know what this black goop in the subway is?” (comment), Reddit, May 27, 2016
2016 (earliest)
Workers walk the line collecting trash tossed down by riders plus other debris that could catch fire and cause delays. The TTC is getting a vacuum car but human hands will still need to rid tracks of “tunnel fur” — a grey cotton-candy-like mix of human hair and skin, rodent feces and oil.
—David Rider, “Subway closures painful but 'worth the wait',” The Toronto Star, March 06, 2016