As of last fall, the locomotive and cars at Cimarron are still in the highway dept. yard, near the highway. You can see it from a distance. There's no telling when the park service will finish any needed rehab on the trestle, much less dangle the locomotive over the trestle again. (When removing the loco, the crane was close to tipping.) But the short drive down the canyon to see the trestle will give you a feeling for the cramped confines of the canyon.
The water tank at Sargents is worth a few minutes, but Marshall Pass is probably a no-no. I've had to turn back on the 4th of July. Monarch Pass, though, does have the old grade on the east side. Locating the switchback can be difficult, since there are a number of 'roads' there to confuse you.
Further east, I suggest a 20-minute drive south towards Alamosa. Take the west turn that leads to Marshall Pass, and drive far enough to see (or drive) the Alamosa/Gunnison cross-over. You can also see one of the short wooden trestles, with rails 'on grade'.
Have fun!