This weekend brings another service closure to the TTC's Line 1, the new official name of the Yonge-University-Spadina subway line. The line will be closed Saturday and Sunday between Bloor and St. Andrew stations.

Worse for those who head into the downtown core on weekends, similar shutdowns will happen for the next three weekends, ending on the weekend of April 5-6. In place of subway trains, shuttle buses will operate between the stations.

The shutdowns will also close access from Union subway station to the rest of Union Station. Anyone heading to the Air Canada Centre, for example, will need to walk from King or St. Andrew station or take the 6 Bay bus.

The reason for the shutdown is signal upgrades, a phrase Toronto commuters are used to hearing.

This week the TTC released an interesting YouTube video that makes the case for why this signal work must happen on weekends, despite the considerable inconvenience to passengers.

Fixed Block: the outdated system we've got now

Signal systems ensure trains are kept apart by a safe distance. They're needed because the stopping distance of a subway train is too long to rely on the driver's line of sight alone. TTC trains are currently controlled by a signalling system installed in 1954. Known as fixed block system, this technology protects the block of track the train occupies and two additional blocks: one in front and another behind each train. The TTC says this system is "safe but outdated" because it requires large "headway gaps" between trains. Larger headway gaps means less room on the rails for trains during rush hour.

ATC: The new system

Automatic Train Control, the system the TTC is working to install, works much faster, allowing trains to operate closer together. The ATC system communicates in real time with transit control, allowing for protection blocks that essentially move with the train. The TTC says this allows trains to travel faster and closer together, all without compromising safety. That means more capacity to move passengers.

OK, so why the weekend closures?

The TTC says the old system must be switched off to install and test the new one. This means the work can only be done when the trains are off the tracks. During the week, that leaves only a 90-minute work window that ends before 5 a.m. Working this way doesn't leave enough time to get the work done, which means weekend closure are needed. The TTC says one day of shutdown on the weekends is worth five weeks' worth of overnight work.

When will it be done?

The TTC has pegged 2017-18 to wrap up the work but that's only on Line 1. So far, there's no funding in place for signal replacement on Line 2 (the Bloor-Danforth line).