Drop out of power in a block bordered by two mth switches

Sounds like a bad connection on the drop. Can you test it with a load, like an automotive bulb? A poor connection will show voltage on a tester, but lose connection under load. 

Worth a shot thanks John!. Used to live in the real Upstate myself...

Does a caboose lose power too? Or only an engine stop running? Engine might be losing common.

Thanks Bobby, yes the engine stops, interestingly there does seem to be ground on both rails other than on the switch where you wouldn't expect it but I haven't actually tried using another powered device like a lit caboose of a searchlight car. John made a similar subjection of probing with a bulb so will try that as well.

Thanks everyone for the suggestions. It proved to be a ground issue. The isolated rails used to provide the anti-derailment function in the switches given that there is no power lock-on in between  effectively created a section without a ground. Unfortunately there is insufficient space for a lock-on but by routing a ground lead to the section and soldering it to the rails were needed the problem is resolved.  A contributing problem is unlike the old ctc lock-ons used on lionel traditional track which provide a neutral path to both outside rails the MTH only connects one. Normally this isn't an issue particularly if lock-ons are on both sides of the track but when caught between switches it can become an issue.

Anyway problem solved with bobby and John's help.

Good troubleshooting! This only happens when the switches are placed with the diverging tracks as opposite turns. Then, the common is lost between the switch turns.

We soldered common rail jumpers on a lot of tracks for a layout restoration. MTH does make a "ground track" 10" straight. I also suggest that multiple lock-ons be used on opposite sides to provide common on both rails.  But, that still doesn't solve the problem caused by the non-derail rails between switches.

The common feed was the easiest fix.