No, take the bus up, and ride the train down. Riding the train uphill is when the engine has to work the hardest, and creates the most exhaust. There's much less soot on the way down.If you don’t already have tickets for the Durango-Silverton train, it’s tough to get them now but you might get lucky. Choose the bus for the return ride rather than the train, IMO.
Riding the end car ("Excursion car", IIRC) on the way down the mountain is best. Especially since that car provides snacks and beverage.