A flywheel (Athearn's famous for this) smooths overall performance of your locomotive when changing speeds, also it makes a trip over dirty spots in your track much easier, because it doesn't jerk to a stop. Depending on the size of the flywheel, you can have a VERY nice running locomotive! :thumb: If it's the N scale FP7, It does make a big difference on handling and performance, and I suggest you only buy locomotives with flywheels, it's like sound, after you own one, you want every engone you own to have them!
(on another note, it seems I'm the first to answer mpost of your questions, Grotto!
Just doin' my job
)