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.

AC compressors can become clogged with tunnel fur (debris like dust and grime) causing AC failures.

I believe that's what the TTC refers to as "tunnel fur"

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.