As you say, The D&SNG seems to function with 5 working engines and the C&TS with 4. Both roads could use an extra engine getting repairs in the shop, but it is not crucial to their operations. BTW, I hear 473 will get repaired eventually, and there is nothing seriously wrong with the unit.
A gaggle of display engines went into restoration around 2000-too many frankly. The good news is C&S #9 , RGS #20 and D&RGW #315 and 346 will run again soon, Alas, D&RGW #169 and the ATSF 2900 at Pueblo were torn down, and the money was not produced to restore them (as far as I know). C&S #74 had problems beyond repair and will return to display.
I would like to see a staggared restoration of various locomotives. The restoration of an engine to operation will add another 50 years to the unit after it goes on display. Also, the introduction of different locomotives at differnt times would sustain interest in railroad preservation.
My dream engines for operation?
I would like to see Nevada County Narrow Gauge #5 run again. I will look forward to RGS 20 running again. #169 is another favorite. Seeing C&S #60 or 71 under steam would be interesting. The Quincy and Torch Lake Brooks mogul up at Huckleberry in Michigan would be worth the trip. Most of all seeing #223 or 268 run again would be rewarding. So many engines, too little moeny!