America's Hobby Center is at 263 (I think) W. 30th Street (definately W. 30th, between 7th and 8th ave., I believe; address is 263/283, something like that; on the north side of the Street). They moved to this new address from 22nd St. two months ago. There phone # (I was just there this evening and have a receipt in front of me) is 212-675-8922. Lots of radios, motors, etc. Good selection of plastic model kits too.
For trains and plastic models there is the Blue Cabose on 45th between Fifth and Sixth Avenue (mid-block), with yet a new train store just across the street.
For plastic kits, the best place in the city I've found is Jan's Hobby Shop in the Upper East Side on York Ave. (way East Side) between 82nd and 86th, somewhere around there. Walk straight out the main entrance of the Metropolitan Museum of Art and follow the street before you all the way down to York, go left, and there is Jan's. Along the way stop at the kite store at the northwest corner of that street and Lexington (I think it's Lex.) Ave.