60103 is correct. Here's a little test. If you have a dual mode loco (runs on DC or DCC) place it on a short track powered by DC. Start the loco. As it travels down the track throw the reverse switch on the DC power pack. The engine will continue to run in the same direction it was traveling. To reverse it you must shut the power off, then throw the reverse switch on the power pack, then turn the know to feed power to the engine. What's happening here is the change in polarity is compensated for by the decoder. The decoder knows it was heading 'east' so it continues east. Whent he power is shut down then the decoder waits for a new command, in this case a change in polarity, which tells the decoder it should 'go the other way.
In otherwords, you should have no problem with your siding being powered the same as the loop. But, if you feel better about not switching the polarity on the siding then insolate it and wire it with feeders from the main bus.