D2 has been back in the news lately, as DART recommences study on the proposed 2nd downtown line.
(Disclaimer: I worked on phase 1 of this study back when the proposed route alignments were whittled from 8-ish to 5-ish)
The need for the line originally was to avoid the presumed congestion and backup on the one current line through downtown as new lines (orange and green) came online. However, planning, design, and urgency was shelved for what other reason? You got it, lack of funding. At the time the proposed routes (IIRC) ranged between $300m and $600m. There was also an issue of lack of consensus, but that is not for me to detail.
Since it was shelved, DART had to rethink all of their schedules and headways (the time/distance between trains on a given line) in order to allow all of them to fit on one corridor through downtown. This often meant less frequent service at other parts of the line. I used to live along the line and can tell you that even with all the timing and headways altered, once the green and orange came online, there is still often a backup of trains through the one existing Bryan Street corridor during rush hours (though I no longer am awakened by 'ding ding...ding ding' at 6am every morning -- but that was the least of my problems with my old residence).
So now, DART and its 3.5 (red, blue, green, and orange which becomes red) currently operates like this:
D2 and its various alternatives propose to create something like this:
The line would have to go subterranean in order to cross the existing line as it runs from Victory and the Perot to somewhere near the Convention Center Hotel and City Hall, which happens to have a vault pre-constructed in case a subway were ever built -- pretty interesting stuff. Though the exact alignment is still up in the air and I've only roughly approximated some version of it above.
Though, the question begs, must we always put lines to ONE specific destination. You can walk two or three blocks. In fact, if you study development around transit lines long enough you begin to realize the greatest value isn't being immediately on the station but a few blocks away. Why? For the most part rail also tends to be disconnective of fine-grained local fabrics, though not always. Subways aren't, but underground stations aren't always the most desirable and tend to lack 'eyes on the street' and the perception of safety.
However, two issues still remain: 1) consensus and 2) how to pay for it. The reboot started due to a $700,000 grant from the feds. However, the price tag is now apparently somewhere between $500M and a singular, cool Bill with a B. Yowza. As is pointed out in the article, in all likelihood it's 15+ years away from approaching fruition. From doing the urban development capacity studies around the new station areas won't be enough to leverage the line either.
So for whatever reason recently, I remembered being on the occasional green line circulator during special events at the cotton bowl when all of our DART riding worlds' get turned upside down and trains START GOING THE OTHER WAY. Trains ran in a loop past their train hotel where they spend the night, through the Cedars, past Union Station, then back through downtown and Deep Ellum. It was like the rotation of the earth suddenly switching in my mental map of the city.
Yet it took me 18 months to put two and two together because I'm slow like that sometimes. If we need a 'reliever' route, and we can't afford the subterranean proposals on tracks and rights-of-way that don't exist, and we have a operational track through an existing corridor, and that area could badly use increase activity and investment attention, why not expand the notion of downtown? Deep Ellum, Fair Park, the Cedars, and downtown more intricately tethered together (though there remains the pesky aspect of those inner-city freeways ruining all the fun and theoretical vibrancy).
Why not run the reliever route on the existing track, like so:
One thing this does is that rather than using the existing trunk line on Bryan to handle four lines, it relocates that 'load' or pressure onto Union Station, which just so happens to be able to handle it with the 7 existing tracks (though only a few are built for DART trains currently).
Or, rather than cutting Deep Ellum down to one line, green can maintain its current run through Deep Ellum and downtown, while the one oddball, orange, which shares track for most of its run is the only one that takes the southern detour. It's worth noting Orange is the line headed for DFW airport and therefore the most important one to have a Union Station stop. Yes, the Convention Center and Hotel are close enough to Union to allow Union to be its stop. The hotel is a full 800 feet from the platform. That's .15 miles. That's 3 minutes. That's a shorter distance than you'd walk just getting from your room to outside of the S.S. Omni.
[STOP LETTING CONVENIENCE OF ONE TRUMP CONVENIENCE OF ALL. Forgive the yelling, but that is the antithesis of urbanism.]
Like so. Though, this alignment would still have three lines through Bryan corridor, three by Union Station. A compromise? Certainly there will be issues with this alignment, but it certainly seems like the barriers are smaller and cheaper than the current proposals.