1) As Blake has mentioned, those are the usual examples of single track use, and I believe both the Red and Blue line are able to operate single tracking. (Someone can clarify for me)
2) Siemens in Sacramento (where most Light Rail Vehicles and trams are made) does build single-articulated trains, but the S70 (a tram model sold by Siemens) is bi-articulated and I think it's the exception. The reason we continue using single-articulated cars is for flexibility within the fleet. Since trains are 4 cars in length, we can switch out individual cars if they are needed for maintenance or if they break down. In cases where a car breaks down, they will usually remove the car that is broken if its the lead or tail car and let the train set operate as a 3 car train (This is based on my experience witnessing such events, so someone who's more knowledgeable can add on to what I have). It's just more easier to operate the same style and it avoids issues where an entire train set would go out of service if it was a full 4 car train permanently. Plus, we don't have the facilities to provide maintenance to cars longer than the standard design right now. I'm not sure if the S200 can be redesigned for additional space, since only Calgary and San Fransico MUNI operates S200s.
3) Our U2s are very old, and are much more worn-out than Edmonton's U2s. We operate them more often than they do, and our U2s are exposed to the weather much more than Edmonton's (remember that Edmonton's Capital Line runs like a subway underground in downtown, while ours is street level). Due to these factors and many others, the U2s in Calgary are much more older looking and rusting much faster than Edmonton's. The city did look at the possibility of refurbishing them, but didn't go ahead and it was deemed cheaper to get new cars than refurbish the old ones.
I should point out that our SD-160 model is getting refurbished as we speak! We have no details as to when it will return from Sacramento, but in case you want to read on the thread, here you go: