Way back in 1993, we ran a weekend of charters on the C&TS and were able to operate with 3 locos without spark arrestors. This was after the rotary run, and before the season opened.
Still, we set a couple of fires in the Narrows, as 488 struggled to get a train started for a runby in the afternoon. A C&TS employee(Chris), was along the highway, and put them out for us.
The logic was that it was primarily moist from snowmelt in the region. However, I doubt you will find many other photos of trains operating without them since then.
Greg