Is it possible to get to Atlantic City from London, Ontario by train?

Answer 1

Yes.

New Jersey Transit trains link Atlantic City with Philadephia.
Any number of Amtrak trains connect Philadelphia and New York city.
From NYC, there is one daily train to Toronto.
I'm not certain if one can get off in the Hamilton area or nor, but either there or Toronto one can switch to VIA Rail Canada to continue to London.

Answer 2

If Canada's passenger rail system has a connection from London to the nearest Amtrak station (which, I don't think they do) then theoretically, yes, one could. From that point one would need to take an Amtrak connection to Philadelphia and then hop an NJ Transit train to Atlantic City.