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This device detects if heavy objects have fallen off the bridge via the delibrate breakage of a wire. It is connected to the signalling system. The trains will automatically stop and will only proceed once the control center has visually confirmed that there is no obstruction on the track (using the CCTV cameras). If the object is on the track, maintenance staff will have to go there and remove it before trains can proceed.
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Thanks for the insight, @James ! 
This is the equipment access patch that allows track equipment to leave the site. They have a similar one right in front of Nuns Island station
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This is one of several permanent points on the network where maintenance vehicles will access the track:
- At each of the 3 depots
- Ile-des-soeurs station just before the bridge
- Gare-Centrale just before the tunnel portal
- Between the Mont-Royal and Côte-de-Liesse stations - Rue Jasper
- Near the A13 highway - Rue Thimens
- Near Ste-Dorothée station - Rue Antoine-Dalmas
Any others you might see are only temporary construction accesses
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Une belle ligne droite pour des pointes 90 km/h… Attachez vos tuques!
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What kind of heavy objects can fall?
Objets qui se détachent d’une remorque, débris d’un accident
Somebody jumping…
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I didn’t want to say that but it was my initial assumption…
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That’s a highway overpass. They have different fences for pedestrian bridges.
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If it was for something else, considering it’s a highway overpass, the fence would have to be longer than that. A high speed object falling from a car or truck could easily pass before the fence and end up on the track.
Oh… 
Le mur antibruit dans les premières photos est installé depuis plusieurs années déjà. Cette année, on prévoit la construction de la suite de ce mur vers le sud jusqu’au mur au nord de la station Canora.
Source: Travaux - Mise en place du mur antibruit | REM
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Il y a un peu plus de détails sur la reconstruction des deux passerelles sur le site info-travaux du REM, notamment sur les échéanciers des travaux successifs.
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