These questions and more are answered by Scientific American.
The complicated question, how many drowned and how many survived, is probably impossible to answer. But we can speculate using the information and knowledge we have in our possession. But things we really need to know, we don’t – information is just not (and some of it never will be).
There are many ways up to the surface, but they all go up. And if the water was mainly gushing into the tunnels from above, from the streets as Sandy was flooding, they would have had to swim or dive up narrow pipes, essentially vertically up against the No way. Those guys drowned.
Rats already on the surface would probably be fine. The water and wind from Battery would carry them north until they reach the dry ground.
The author guesses most survived.
Sure, many of the city rats are HUGE. Why? Their diet is good. Will they take over now that the weaklings drowned? (As we saw above, they probably didn’t all drown.)
Will these large African Rats takeover New York finish what Sandy started?
I don’t think so. We still aren’t 100% sure that these are Gambian rats in the projects, though I think they may be. To be 100% certain we would need a specimen to inspect, get some genetic sample and ideally voucher.
Yes, these rats are good climbers and are likely to be good swimmers, too. Assuming there are African Pouched rats in New York City, they are survivors, but it doesn’t seem there is a viable population to threaten the safety of the general public.