just an opinion here, i think if we build eglinton, there will be no choice but to build a DRL after it. but if we build the DRL first, the DRL doesn't put pressure to build eglinton. eglinton will add to the reasons to build the DRL but i don't think building the DRL will add to the reasons to build eglinton. doing eglinton first will give us the greatest chance of having both in the near future.
Even without Eglinton, the Yonge line is already rammed. And it's still going to be fairly busy, even after they roll out TRs on it. So I don't know how much one will impact the other.....unless you think the only way to get politicians to commit to the DRL is perpetual overcrowding at Yonge-Bloor. I'm hoping not to be that pessimistic yet....but you could be right.