Question: What are the extra rails for?
EMS has already done this one, but I don’t remember where that post was – if I can find it I’ll add a link.
Passengers looking out the window will sometimes notice (especially along the Banfield) what looks like extra rail laying in the track. It looks a bit haphazardly placed along the Banfield so it’s not as obvious what it’s for as it is when you see it placed symmetrically, like in the above picture which is the overpass just east of Sunset TC.
These guard rails are placed as added protection in case of derailment in high speed areas. Granted, derailment isn’t good under any circumstances, but there are some areas where a derailed train would be more catastrophic than others. So you’ll see guard rails wherever trains go over an overpass, like above..
…or under them, like here at the NE 82nd Avenue platform. In both cases, derailments would be extremely dangerous and could potentially cause serious structural damage to the overpass – to say nothing of the damage to the people inside the train – so the extra rails are placed as added protection should the wheels leave the rails they’re supposed to be on. Derailment along flat terrain is obviously not ideal, but a train derailing into or off of a bridge is far worse.
Or doing this…
You can also see those extra rails placed around curves, like in this picture looking west from Beaverton Central, or like this picture I’ve already posted looking east at Gateway. All of these examples are in t-rail, not girder rail.
Trains can still derail in girder rail, but guard rails aren’t used in that type of rail – I think both the physical structure of girder rail and the low speeds that trains travel through areas with girder rail wouldn’t benefit from adding in guard rails.