Probably evacuated to philly... because of existing infrastructure...New York is out...there were riots I think at one point. Afterwards it remains temporary while new ifrastructure is developed further west more in the centre of the New Union. of course if they cannot agree....Who is to say that temporary would not become permanent.
Aye, if New York draft riots then that probably takes it right out.
Any accommodations in Philly will be temporary in intention, and thus probably very far from ideal. We'll have the bureaucratic inertia that builds up as more and more government agencies set up shot in Philly, balanced against the desire of the governing officials themselves to get out, or back to the place that was designed from the get go to be the capital (DC).
Now, could they go to Philly and set up shot some ways outside the city, and build a whole new Capital District from scratch? Yes, and that's what I'd pick if I couldn't pick DC.