What's the Difference between the P70/72?

What is the Difference between the P70/72? I know the P70's use to run in Chicago on the Valpo Commuter.

John

The difference, as taught to me in 1st grade, is 2.

Seriously though, railroads and car builders give different classes to cars/locomotives and also redesignate classes as well. There are differences in appearance and mechanical capabilities as well. I'm not sure if the P70's had generator plants, or if any were built with roller bearings.

Jay