I don't know about right behind the house, but I think there is an old rail line at the very bottom of your screenshot. If you use Google Streetview (the little orange guy) and walk him along Gairloch Rd to the southwest of the house as you have marked it, you can see where the rail line crosses. It is after Lansdowne Station Rd, but just before Lansdowne Rd. It is 500 metres from the house.
These are the coordinates for where the rail line intersects Gairloch Rd.:
45.42684, -62.82989
Google maps link:
Street View · Google Maps
If the kids actually walked along Gairloch Rd (something which I think we were told they would *never* do - walk along or cross Gairloch Rd) they could have reached that rail line and then walked along it.
If you turn right off Gairloch and follow along the tracks, the rail line crosses what is being referred to as "the pipeline trail". It is only visible on google maps when you have the satellite view (not just the map). It is roughly parallel to Gairloch Rd. It runs behind the house, probably just a bit more than a kilometre back. And yes, it looks pretty dense between the house and that pipeline access trail.
Where anything I said might be construed as an opinion, it's MOO!