for MPC, a good rule of thumb is the engine always started in 8 (for the most part), the next number would stand for the year it was built. The other 2 just seem to be what ever was next in series. There were times when Lionel would only put the prototype number on the engine & the catalog number would not appear.
For example - the 8600 NYC Hudson, the 6 = blt/catalog date of 1976
The rolling stock gets a bit more complex, but off the top of my head -
9200 - Boxcars, Tank Cars, Operating Boxcars, Large Hoppers, B/W Caboose
9800 - Standard O, Billboard Reefers
9700 - Boxcars, Large Stock Cars
9400 - Boxcars, Large Stock Cars
9600 - Hi Cube Boxcars,
9300 - Gondolas, Crane Cars, Tank Cars,
9100 - Flatcars, Cabooses
9500 - Passenger Cars
9000 - Small Hoppers, lower end SP caboose, small flatcars
6300 - Gondolas
6000 - Small hoppers
7000 - Some Mint Cars
I know there are more, but that's all I can get out of my brain right now.