Behind a city that has grown with the times, sits a remarkable system of handle-pulls, audible clicks and banks of early 20th century electrical technology – all housed in three castle-like downtown towers. Combined, it has constantly shepherded trains in and out of Canada’s largest city. Designed in the late 1920s, the complicated contraption is about to be retired – though it could have lasted, arguably, a century more.

Inside the long metal box, electric synapsis fire like neurons around your head.

The locker smells like the inside of granddad’s old tool case. And while perhaps as long as a typical one-car garage, you can’t extend your arms fully out from your sides without disrupting important gadgetry and vital currents moving through coils of wire that snake through holes, before dropping into the room below.

Once at that lower level, electricity races to banks of glass K-type relays, mounted on thick wood panels that were once regularly oiled to preserve their life and shine. Every wire has a path running through holes listed alphabetically and by the numbers, before connecting into a puzzling maze of more energy panels.

And from there, those sporadic pulses move outside three old brick towers built in downtown Toronto to 180 signals and 250 track switches that dictate the movement of 900 train trips – from GO to freight to UP Express to VIA Rail – travelling daily along the 6.4 kilometres that make up Union Station’s rail corridor.

The box, and all the collateral wiring and apparatus, is part of four electro-mechanical interlockings. It’s 1920s technology that includes, on this day, two Toronto Terminals Railway (TTR) train movement directors spending their shifts inside the John Street tower, pulling and pushing black and red levers that line the outside of the box. As they do this, they listen for double-clicks that signal a lever is in the right position to send a train along the correct route.

The levers have been worked, 24 hours a day, 365 days a year, since it was put into service in the early 1930s, after being built in the late 1920s. It must have seemed like a technological wonder at the time – because it still is.

Though, recalls Vito Parisi, who started his career watching over the interlockers 35 years ago: “I just thought ‘this is a rat’s nest (of wiring).’

“It was something that I had never seen before in my life.”

A rail interlocking is a failsafe system of signals and switches that prevent the wrong movement of a train along a route. It’s an invisible steering wheel. Even back when Parisi began, this elaborate system of putting trains on the right track was rather ancient. Though, he notes, it was also – having been tried and tested over generations – brilliantly designed and masterfully constructed.

It remains almost foolproof in its functional design.

“You won’t learn it in one week, or in one month,” Parisi says of his career with the network of machinery and electrical signals. “You never stop learning about it.

“It’s relay logic.”

Today, semi-retired Parisi still toils around the interlockers as a signal consultant, and is intrigued as Metrolinx installs a new microprocessor based interlocking system to control the entire corridor within the next few years. That means the interlocking being directed from the watch-towers will soon finally all stop.

The change has to be done systematically, as the existing three towers full of 1920s technology continue to buzz and click along behind the scenes. Experts say it could, other than being unable to handle an increase in traffic as well as difficulty manufacturing replacement parts, have continued to technically move trains for perhaps a century more.

The truth is there are few people alive who could build something like the existing electro-mechanical interlockers system, which saw many components imported from Rochester, N.Y. It’s a marvel of another age, and, where once most major rail corridors worked with very similar equipment, the Union Station system is perhaps the last of its kind in North America.

The needed modernization, including remote access points to quickly collect diagnostic data, will involve 258 track circuits, 35.4 kilometres of conduit and more than 305 kilometres of cable.

For customers, the work will mean quicker speeds within the corridor, as well as spare capacity for future expansion.

For the box, it means the end of the line and times.

Dubbed ‘the pit’, workers step down about 76 centimeters to enter the locker box at the John Street facility. As those on the outside pull and push the large levers, watching a map on the wall for telltale lights that tell the story of routes through the heart of Toronto, various mechanical slots fill and empty with metal rods, depending on choices made. Think of it as a puzzle box – as choices are made, and pieces move, some options open and others close. Within the aging brilliance, these workings still protect trains from bumping into one another on the tracks.

“Other than perhaps the lights, not much has changed in here since the first day it was built,” says David Kolbasovsky, Metrolinx manager of signals and communication, as he moves through the pit.

“It’s extremely safe and a lot more reliable than you would think. This was the Elon Musk of (the 1920s).”

The new system will be shifted out of the towers, including the John Street facility. As a heritage building, it can’t be properly upgraded. A sentinel beside the tracks, it’s still uncertain what date it will finally stop sending out its electrical impulses.

That day – along with the change-over of the other two towers – will likely happen without GO Transit and UP Express customers noticing. Their journeys will suddenly be guided by computer chips and data streams, rather than mechanical handles, clicks of an outdated language and amp meters fluttering along with the currents.

The change will coincide with other, separate but connected improvements and advances.

Metrolinx does not dispatch trains right now. As the interlocking system is being brought into the digital age, the transit agency is preparing to dispatch all GO and UP Express trains using the ‘GO Transit Train Control System’.

It’s unclear what will happen to the locker that’s been in service since 1932, along with the unique walls of relays clinging to their wooden supports, across a system spread out from the three watchtowers. Some of those relays haven’t been taken down off their supports for an overhaul since the 1950s.

But for now, they keep time for Canada’s largest rail passenger facility – as they’ve done for as long as anyone alive can remember.