There is a whole synopsis on this. I had the thread book marked but not sure where. short short version=
200 and 500 series cars are standard gauge
600/2600 and 800/2800 series cars are O gauge. 600/2600 are smaller and shorter than 800/2800 series.
Engines don't follow the above convention. I will try to follow up later, maybe somebody else can dig up that link with the explanation.