Until about 15 years ago, model trains were powered by “direct current” -the same stuff that comes from flashlight batteries. A “transformer” or power pack converted house current (AC) into DC, and fed it to the tracks. The amount of power being fed was controlled by the throttle (speed knob) on the power pack. No power -train doesn’t move. Full power: train may fly off at a curve and land in your neighbor’s outdoor grill. And so, the amount of power applied to the track is what ran the train slow or fast.
Sounds easy enough -until you put more than one loco on the track. They can’t be controlled independently because they both are controlled by whatever power is applied to the track. One throttle is powering 2 loco’s with exact same power setting at the same time. And even though they both get the same amount of juice, one loco will go faster because it has a different motor in it, or maybe they’ll run in opposite directions. Bottom line: it is a hassle. The solution is to put gaps in the rails so you have basically two isolated portions of track -and get a separate power pack for each section. But that gets complicated -and expensive.
But these days, there’s a way to get around the problems of plain vanilla DC. Apply FULL power to the track ALL THE TIME, and THEN put little micro-chips (called “decoders”) in each loco that control how much power the loco will use and which direction it will go. Talk to the decoders by sending commands to them through the rails. Give each decoder its own unique “address,” so that ONLY the commands sent out to that address will control the loco where the decoder is located. Thus, decoder #1 won’t respond to commands sent to decoder #2; and #2 won’t do what you command #1 to do. This is like a set of remote control garage doors; each one has its own address, so the remote for door #1 won’t open door #2, and vice-versa. Simple concept. BRILLIANT concept, in fact.
NOW you can have 2 or 50 locos on the same track, and as long as each has a decoder with a unique address, you can run each of them independently of the other. The power pack itself has a an array of buttons sort of like a push-button phone -that’s how you select an address. So, you punch in “301” to get one loco going, then punch in “401” to start another -and meanwhile, 301 just keeps doing the last thing you told it to do. And, as long as you’ve got decoders for the locos, why not use them to throw switches and do other things on the layout as well? Exactly -you use “stationery decoders” for those tasks -and they get their power from the rails, too -same as the locos.
This is all very well and good -but what about the existing investment people already HAVE in plain DC operations? The answer is to retrofit existing DC locos with decoders, so they will now run on a DCC powered layout. And as a matter of fact, most DCC systems will allow you to run ONE (just one) DC loco while you also run DCC equipped locos at the same time.
But what about the old style DC transformers -will they work with DCC equipped loco’s? The answer is, “well, maybe.” Some DCC power packs will allow a regular DC transformer to be “slaved” to the DCC unit so it can control one train. But generally, your old DC transformer can’t be deployed to run DCC trains because it has no way to “talk” to the decoders. It CAN however, be used a source of power for other things -such as lighting or small DC motors which run accessories -like a merry-go-round or a crane.
The wiring for DC and DCC is basically identical -two wires go to each of two rails; and so, you can usually swap out a DC transformer with a DCC unit, hook up the existing wiring -and everything should work.
There’s more to it than that -but now you have the basic idea. Search the ‘net for model railroad DCC -and start your research.