Tortoise machines have an internal resistance of about 600 Ohms. You can use them in series with red or green LED's. We have a panel that uses red/green bi-directional LED's (basically a red and a green in the same package wired in opposite directions. Using the AC-source diagram below (which we used at the club), just put the red/green LED in series with the machine and it will indicate the position. By the way, setting up an indicator trackside (safe from being sheared off by low-hanging rolling stock) is a great idea which in hindsight I wish we had done.
