The entire wheel/bogey assembly is built from from five or six smaller pieces. There are
a lot of guys carrying the wheels themselves.
The RC engines run on batteries and are not quite as powerful. Apparently the batteries only last for a few hours, which makes them impractical for train display shows (the AC powered engines could in theory run forever). The IR range is also fairly short, something like 10 or 15 feet, and you have less room to build on the engine due to the big battery block in the middle.
They're supposed to come out with a new system in 2009 based on the power functions line introduced in Technic, but it will most likely run on the same tracks as the RC system and will be battery powered.