Does anyone know of a good website or the equation used to find the thrust, preferably in force (newtons) that an engine puts out when turning a certain RPM and propeller diameter? I'm looking for this information for a physics project in which we will have a magnetically levitated train propelled by a small electric (brushed) engine. Thanks guys.

If you will convert the thrust in lbs. then this link will get you a nice thrust calculator program:
[link=http://www.bmaps.net/software/thrusthpv20d.zip]RC Thrust Calculator[/link]