For motion you have one of two options. The Cruise Commander gives TMCC control and constant speed. The DC Commander gives TMCC control but speed will vary with load. Either will drive one large motor typically found in Premier Steam, or two smaller motors used in their diesels. If you have AAs with four motors you should get a separate motor driver for each engine. Some have gotten away with one board driving 4 motors but that leaves no room accidents causing wheel lockup.
For sound you need to add the Railsounds Commander.
Also if you are upgrading PS2 or PS3 engines you have to replace the couplers. proto 1 couplers are compatible.