Sierra; CL2 are current limiters for LEDs. It is a two terminal current regulator, 20ma output with 5 to 90 volt input. Google CL2N3. 44 cents each from Mouser 689-CL2N3-G With these you can use any DC power supply regulated are not. You also can use current limiter resistors with a regulated supply, just tie the negitive side to the negitive side of the track power so the two rails will be the returns and witch one is negitive will give you direction. Run the positve of the supply out to the lights and hook each one to a rail using either a current limiting resistor or the CL2. You can still use your AC output for the 2nd supply. Its old school and simple.
You should not use your varible DC output for accessories like that but a few LEDs would be OK. Most of your accessories will be AC anyway. AndyTrue it might seem like overkill, but I anticipate that the layout will grow and I could use the extra power for other needs like uncouplers and machinery that might need a DC power source