You can have a separate track, and you can also quite seamlessly blend the sensor track into Atlas track. Here's the job, the ballast was just tossed down to see the effect, it will be done more permanently. It's actually quite easy to slip it into the Atlas layout. It happens that it's the exact height of the cork roadbed and Atlas plastic ties, so just taking the Fastrack rails off and knocking off the little plastic nubs, the roadbed slips right under the track. Obviously, the Atlas ties were removed as well.
