It isn't there currently because the standard we're working on is to not include peak hour only services that run entirely concurrently with other regular services. This is why for example you also don't see the Z train in New York, or the yellow line fork to Franconia-Springfield in DC.
You can mark the route of the train you took traveled via the Brown and Red lines, which you would get credit for anyway as concurrent if the Purple Line were extended to the loop and you marked it.