Every operation needs a diesel for back up. The only exceptionS might be Cass Scenic, Durango & Silverton, and Combres & Toltec. I don't think any of those operations have diesels right now that can make take a train over their full route. The Durango & Silverton has plans to buy larger diesels but they have not arrived, I don't think their current diesels could candle a full train up the grade.
Strasburg has run diesels as back up a few times over the past decade, making a diesel powered trip rare enough to be considered special by most railfans.