Timing is an issue. They still need to get freights through. I long to see the day capacity requires 8 double deckers.
Dallas underestimated the popularity of DART and thus some lines will never accommodate more than 2-car trains.
Never say never; I've seen large sections of light rail lines rebuilt at massive expense less than 10 years after initial construction, simply to lengthen platforms. You can expect Dallas to do so at some point.
Actually, speaking from memory here, the underground sttaions on DART do seem to have long platforms, so were presumably designed with future proofing.
Extending the on surface platforms shouldn't be excessively expensive (a handful of special cases aside - such as the stops on bridges or viaducts).
In this respect one shouldn't forget that everything has an expected or nomibal lifetime. So accountants will depreciate the cost of, say, a platform over 30 years. If you are expecting a longer platform to be justified after say, 25 years, then it makes more sense from an accountancy point of view of making do with the short one iniitially and then going for a longer one when the time comes.
With the oldest parts of DART now well over 20 years old and slowly beginning to feel the squeeze, I don't think its appropriate to say the initial system was under-dimesioned. Rather, it (or vital components thereof) are reaching the end of their designed lifecycle and presenting an opportunity to be rebuilt in a more high capacity form.
