Henry,
Since you haven't gotten any responses, I'll take a crack at it. In the past, I probably would've said don't worry too much about reservations for the morning steam excursion on the NN, but...
Things are changing in Ely and the NN is slowly being discovered, thanks to the efforts of the current director and his team. That trend is likely to continue this year, as the NN Ry Museum moves its 2 year Centennial celebration into high gear.
To be on the safe side, I'd see about making reservations as your date draws near. Be sure you've got a place to stay as well - Ely is a prime stopover point (with few/no alternatives in either direction for hundreds of miles) for travelers along Hwy 50, the so-called "Loneliest Road in America", and accomodations can fill up rapidly in the summer.
I'd also try to at least leave enough time to take a tour of the yards and enginehouse. The extensive East Ely facilities are the heart and soul of the RR. East Ely is to standard gauge shortline RRs what Chama is to the Narrow Gauge (obligatory NG content
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With apologies for the non-NG content...
Scott