Can a Lionel SC-1 be used to turn on and off an accessory that runs on DC? I don't have any switches connected to the SC-1.
Original Post
The short answer here is No.
The SC-1 uses a part called a Triac to control the flow of electricity. Triac's are perfect for AC power, but almost useless for DC because they can not be turned off if current is flowing through them. With AC power the current reverses polarity 60 times a second (in the US, anyway), thus the Triac has a chance to be turned off 60 times a second.
If you had SC-1's laying around you could use one to trigger an AC coil relay, and then use the relay to switch your DC device, but using an SC-2 is probably a better choice as it already has a relay inside.
Ine last option, again, if you already have a pile of SC-1's that you don't care about messing up could be to modify one to operate DC devices. I'd have to get inside one to see exactly what would be involved, but it seems you could replace the Triac's with MOSFETs or transistors to conduct DC in a controllable manner.
JGL
