If I read your troubleshooting correctly, it's the loop at fault then. Does this loop have multiple feeds (blocks)? Remove them one by one?
Is there something on the loop or connected that could draw down the signal? Like switch motors, lighted cars or buildings, or TMCC equipment?
Did this loop work originally? Did you do anything to change this loop? Have you removed everything from the tracks for a test?
If they don't receive any signal, the engines would start right up when they see power. If they don't and stay quiet when the rails have power, they are receiving the watchdog signal at least.
