Is there a train that goes from YYZ to EWR / somewhere in NY state?

A British passport coming in to Canada should be no big deal.

You can consider flying from YYZ to EWR. There are at least 2 airlines that fly this route - Air Canada & Porter. (I'm not sure if United / Continental fly this route or if they just codeshare with AC.) If you fly AC, you'll depart from the main airport (Pearson International) which means you actually go through US Customs here in Canada. When you arrive in the US, you land at a domestic terminal so you just walk off the airplane with no further fuss.