One glance at the disgruntled expressions of TTC riders boarding Yonge St. shuttle buses and it’s quite clear how they feel about subway closures.

What’s not quite as obvious is the reason for the two-day closure of a section of the Yonge-University-Spadina line.

This weekend, more than 50 people will work around the clock beneath College and Yonge Sts., installing a track feature called a double crossover.

A double crossover is a set of switches that link two parallel railway lines. It appears like an X in between two sets of tracks, and will allow northbound trains to cross over to the southbound line, and vice-versa. The system already has 19 double crossovers; the one being installed south of College station will be number 20.

Jim Teeple, Toronto Transit Commission’s superintendent of the track and structure department, and Danny Nicholson, with TTC corporate communications, broke down the subway closure for the Toronto Star.

What’s closed?

All stations along the Yonge line between Bloor-Yonge and Union stations — that’s Union, King, Queen, Dundas, College, Wellesley and Yonge — on Saturday and Sunday. Regular service resumes Monday at 6 a.m.

That means trains on the University-Spadina line will turn back at Union, and those on the Yonge line will turn back at Bloor-Yonge.

Throughout the closure, the TTC will run accessible shuttle buses between Rosedale and Union stations, stopping at Bloor, Wellesley, College/Carlton, Dundas, Queen, King, Front and Bay Sts.

The TTC is also suggesting riders walk or take the streetcar to University Ave., and travel north to St. George station, then ride the Bloor-Danforth line to Yonge. They can then transfer back to the Yonge line for northbound service.

What’s going on?

The TTC is installing a double crossover.

A crossover is a pair of steel switches connecting two parallel tracks, enabling a train on one track to cross to the other. A double crossover connects two tracks to each other, meaning a train can switch tracks in both directions.

“Think of it like a pair of scissors that will sit between the two existing tracks,” said Teeple.

The change won’t be visible from the platform. Trains will only make use of the feature in the tunnel south of the station.

It will come in handy when there’s a delay or emergency, such as a disabled train, signal problems or a broken rail. Trains will now be able to change tracks at College station and travel in the opposite direction.

Before this installation, there was nowhere for trains to reverse directions on the Yonge line between Bloor and Union, meaning if there was a delay at any of the five stations in between, trains (and passengers) would be forced to sit idle.

Nicholson said a recent broken rail at King station is the kind of incident when the College double crossover would have been useful.

College station was selected instead of Dundas, the midway stop, because the tunnel structure was better suited to the installation.

How will this affect riders?

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Teeple says passengers will probably not see big improvements right away. The change is part of a larger project to install an automated train control system, expected to be completed by 2014.

When will the work be done?

There will be one final weekend closure to finish the job, slated for the March 26-27 weekend.