While it is tempting to say Yes, this may have unpredictable results and may even damage some equipment.
My recommendation: Keep things separate
If you want a layout that supports the running of DC locomotives as well as DCC locomotives, there are essentially three options available to you, with varying degrees of hazards and complications that you will most likely encounter.
Keep things Separate
The safest and easiest approach is to run separate tracks or blocks for your DC and DCC trains. Many modellers that have made the decision to switch over to DCC, run a single DC main line, with say a few smaller sidings to keep it interesting- the rest is all DCC.
A more complicated approach is to switch between DC and DCC as required. This however may become a wiring nightmare on larger layouts. The DCC system and the DC system are never run at the same time and are 100% isolated from each other. This typically involves some kind of electrical switching system be it as simple as a DPDT toggle switch for a simple layout all the way up to a distributed relay driven switching system for larger layouts.
Most DCC systems allow a single DC locomotive to be operated on a DCC system. If you wish to run numerous DC trains, this will be a major drawback. The only electrical issue here with this choice is the long term heating effect on the DC locomotive itself.
Take a Major Risk and run Parallel
Even if you plan carefully, you risk damaging your equipment when something goes wrong. Running DC (Direct Current) and DCC (Alternating Current) will create a very uncertain situation on your layout where the outcome is uncertain and destruction of one or both system can result!
Ok, you still want to go ahead and try it..here’s how to proceed:
By leveraging your layouts existing electrical track “block wiring” system. (If your layout does not have electrical track blocks, this option will not work.)
To make this work, you replace one of the multiple DC throttles with the DCC system and run the DCC power to the track blocks just like you would for any other DC throttle. You can get 100% DC all the way up to 100% DCC and any mixed percentage in between.
HOWEVER, there are serious electrical risk issue in play here.
While it is true that the electrical blocking system keeps the DCC system isolated from the DC power packs/throttles, this is not guaranteed when you run your trains. Having any rolling stock, engine or cars with metal wheels, crossing between a DC power block and a DCC powered block will place the two system electrically in parallel with each other. Why? A DCC system puts out on the track a special form of AC (Alternating Current) where as a DC (Direct Current) throttle puts out DC on the track!
Electrically mixing AC and DC is NEVER GOOD
Original Article credit: Mark Gurries