My largest curves are 042, and the Williams moves through them with no problem whatsoever. HOWEVER: the only switch / turnout mine will go through without derailing is a PREWAR 072. I've tried K-Line, Lionel, MTH, and the pilot trucks will jump them all. I don't know if it's just mine, or if anyone else has had a similar problem.
No matter, it's still a great engine, and I'm glad I own one.