Actually the old Athearns are among the easiest locomotives to install dcc decoders in. The red and silver model is an Athearn Alco PA.
You are going to need to take the drive system apart regardless because the basic design of the Athearn drive is good, but the use of various slider type contacts makes them unreliable, but easy to fix.
Here is a link to my friend Jim Furhmann's web page on fixing common problems with Athearn diesels.
Tune Up for HO Athearn Drives
The next page is his directions on how to rewire the Athearn diesel with what he calls his 5 wire system.
Five Wire System
When you take the motor out of the frame, you need to isolate it from the frame electrically. If you follow Jim's directions on the 5 wire system, you will notice that the first thing he eliminates is the steel slider contact on top of the motor that picks up power from the top of the trucks. Notice that Jim removes the motor and carefully swaps the top brass clip holding the motor together with the bottom clip. When you do that, and put the motor back into the plastic motor mounts, the motor is automatically insulated from the frame.
The five wire system, is shown for dc, to use it for dcc operation, you don't hook those wires to the motor. Instead for dcc, elimintae the 5th wire and make a 4 wire system. You don't need the wire that connects the frame to the motor. Every decoder includes instructions on how to wire it up, and you should read those instructions first. You should have 2 wires on the decoder that hook up to the positive and negative sides of the locomotive. The wire that should go to positive (this is when the engine runs forward, when running in reverse it is opposite) is soldered to the wires that are coming off the top clip of the trucks. The negative wire should be soldered to the wires coming off the side of the trucks. You then solder the wires that go to the motor to the top brass clip and bottom brass clip respectively. If you are going to wire in lights sound or other functions, just follow the instructions included with the decoder.
On the Mantua steam engine, I think the Mantuas used the same system of power that brass steam engines used where in the locomotive picks up power on one side of the locomotive drivers and the oposite side of the tender wheels. You can use virtually any decoder with a steam engine because you have a lot of room in the tender to hide the decoder. Again the secret to getting a decoder to work in the steam engine is to insulate the motor completely from the frame so that the decoder can pick up power from the wheels/frames of the engine and tender, and then control the amount of power it actually applies to the motor. As far as specidic decoder reccomendations for the steam engine goes, what do you want? Do you just want dcc control? Do you want sound?
