Looking at aerial pictures on GoogleMaps, there is no passing loop there at all. There are 4 tracks, the middle ones have 3rd rails fitted, like the metro would use. The outer tracks don't because they're not interconnected.
There's a cross-over on the metro tracks in the middle, just south of Takoma station. This one is not close enough to the site of the accident to be causing a derailment and have the train slide all the way over to the accident site.
The closest by passing loop is north of Silver Spring station, which is north of Takoma station. So waiting in a passing loop is out of the question as well.
Now it has me wondering "why would that one train stop". I mean, especially when it's a computer controlled system, you would think that there's at least a good few minutes between the two trains. And "did the driver not notice the signal being red?". Perhaps because the computer controlled running of the train day in day out without any noteworthy incidents gets people bored and therefore they're more easily distracted? Who knows. Time will tell, I guess.