Does the lead pilot wheel of your Challenger pick up power from both rails? Have you tried any other steam? I had the same problem with MRC reversers and concluded, but did not prove, that they only work reliably when the first wheelset to cross the gap picks up power from both rails. All my diesels pick up from all wheels, and all worked fine. None of my steam with metal pilot wheels would activate the reverser. I tried to activate the reverser by shorting out the rails with screwdriver tip, this would not trip the reverser, just my circuit breaker. Now, perhaps this had something to do with the ladder of turnouts leading to the reverse blocks (storage loops) causing voltage drop at the gaps, tho I eventually soldered additional feeds to rectify that possibility. When I switched to Tony's units (no longer avail) I no longer had this problem.