Tom
I ask the same question in late summer and did not get a firm answer. We then went to Caliente and while there ask around. I ask a county deputy, he was not sure if the road was/is open all the way through. Google earth lists it as a county road. Part of the local answer is, parts of the county road washed out in the big flood a few years ago and has not been repaired. The UP has an access road thru the canyon but that is on RR property. Off the record I was told the UP will let you trespass on the portions of the county road that is washed out. The gate at Carp was open and looked like it had been for a long time. A bunch of off road four wheelers were in Caliente, I ask them if the road was open thru from Moapa. One guy said, I live in Moapa so I ask him directly if the road was open. I then got a lot of evasive answers so figured he did not know squat. I know I would not attempt it in poor weather or if it looked like it might rain. I would be prepared to back track to the way you came in. I am sure it will be slow going so give yourself lots of time. We have learned to take two spare tires, sharp rocks will get you. Take lots of water. If you learn more please let me know. We plan to give it a try next spring.

Jim in Eugene OR