Many people want the best of both worlds in that the layout can support the running of DC locomotives and DCC locomotives. However it is electrically done there are varying degrees of hazards encountered when doing so. There are 3 ways to do this (rated in increasing risk order):
1) ALTERNATIVELY. NO RISK. The layout runs 100% DCC and then is SWITCHED OVER to run 100% DC and back again as needed depending on the goals of that specific operating session. 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 a large (club) layout. This is the only option that is 100% safest to implement all things considered. Recommended.
2) SIMULTANEOUSLY. MINOR RISK. This is the active running of DCC locomotives side by side with ONE DC locomotive utilizing the DCC systems optional ability to run a single DC locomotive on DCC powered track. The only electrical issue here with this choice is the long term heating effect on the DC locomotive itself. For more information, go here: DC Loco on DCC
3) PARALLEL. MAJOR RISK. This works by leveraging the 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, one simply replaces one of the multiple DC throttles with the DCC system and runs the DCC power to the track blocks just like they do 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. It is true that the electrical blocking system keeps the DCC system isolated from the DC power packs/throttles. But that is not guaranteed when you run the trains. Having any rolling stock, engine or cars with metal wheels, crosses between a DC power block and a DCC powered block will place the two system electrically in parallel with each other. A MAJOR electrical battle will take place the DC and DCC system with the outcome uncertain and destruction of one or both system can result. 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! Electricially mixing AC and DC is NEVER GOOD. To learn more about the differences between DC and DCC, go here: DC versus DCC
POSSIBLE PARALLEL SOLUTION: There WAS only ONE electrical solution that can give you some level of protection BY DESIGN. Lenz offered the LT100. Simply it is designed to protect the DC throttle from DCC by detecting the DCC signal and instantly electrically disconnecting the DC throttle. Maximum ratings: 18V & 3.5A. Go here for more information:
http://www.lenzusa.com/1newsite1/Manuals/lt100.pdf
The problem is that LENZ has discontinued this product. You can check you local Lenz dealer or Tonys Train Exchange to see if they have any left.