As the saw bites into steel rail, a fountain of sparks pierces a dark subterranean soup thick with dust, musty smells, noise and mysterious winds.

On the street above, commuters used to comfortably whizzing through these tunnels while peering at their phones no doubt grumble as they board shuttle buses.

Down here, in a tunnel closed for a weekend of signalling and maintenance work, the man overseeing TTC subways is making the case to reporters that there is commuter gain in this frequently occurring closure pain.

“These weekends are worth their weight in gold,” Mike Palmer says in the tunnel between St. Clair West and Dupont stations, near a plaque marking a 1995 fatal subway crash that reminds staff of the safety-first motto.

“We can do so much more work in a 52-hour closure than two or three hours a night . . . It’s hot, dirty, cold, smelly, smoky. I pay tribute to the men and women who work down here — 366 nights this (leap) year — to do maintenance, good housekeeping, catch up with repairs, and laying cabling.”

TTC chief executive Andy Byford adds that the more frequent closures — this one from St. George to Lawrence West will be followed by two more just this month and next — are actually good news for riders.

Council-approved budget hikes are making possible a full modernization of the system with ambitious targets to improve reliability.

“We need to get our basic infrastructure back up to the state that we need it to be,” Byford says.

“This will be worth the wait because we’re putting (in place) what should have been done years ago — we are fundamentally upgrading the system, introducing in phases starting next year an automatic train control system that will not only transform our reliability but we can run more trains closer together.”

The work being done includes:

Grinding: A heavy yellow rail car rolls along the track, sending sparks flying from the bottom as grinding stones reshape the top of the worn rail. The result, when it meets reshaped train wheels, is a smoother, safer, squeal-free ride. The TTC rents this grinder but plans to buy its own.

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Litter picking: 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.

Rail replacement: Rail too worn to grind is replaced in sections and seamlessly welded together. In most spots they are lifted in place by a crane on a work car. In tight spots, though, 15 workers get on each side of the heavy rail and together lift it into place.

Signal work: Cable and other hardware is being installed for the automatic train control system that will replace a 1954 signalling relic. Yellow boxes contain transponders that will tell trains their location, allowing for automatic speed control and shorter waits for riders.