My opinion whether for CATS or the D&S:
Primarily, it comes down to numbers and cost.
You have to maintain facilities and supply of both fuels. Admittedly, for coal the facilities already exist and as long as a ready supply of quality coal exists at a reasonable price this really shouldn't be an issue.
With both fuels in use, it will be necessary to have sufficient employees qualified and available for either depending on the engines in use. Unless enough are cross trained, this can be an issue. There can some extra costs, but primarily this is a scheduling/training issue. The same situation exists for having diesels and steam.
The biggest potential cost/benefit factor is the FRA required 1472 day/15 year steam loco boiler requirements. 1472 days is four years of daily service, which if your operation is seasonal and you have enough operable engines, you can easily stretch that 1472 well beyond the three years.
To allow an engine to operate up to the 15 year maximum, you would need to use it for an average of 98 days per year to get the full cost benefit of the 1472 expense. Obviously, there are many, many steam locomotives that fall in this category. So in these cases, the cost of the 1472 has to be divided between the boiler operating days within the 15 year maximum. So if you only run an engine an average of 24 days per year, you will only accumulate 360 days of boiler service out of the 1472 available, resulting in your 1472 costs being four times as much per the benefit of an operating day.
So if you aren't going to be able to run an FRA steam locomotive more than a 98 day per year average, you will not be getting the full cost benefit of the 1472 work - assuming no other major boiler issues. Hopefully the running gear and appliance wear is proportional to use.
While history and sentiment are great, you still have to pay the bills at the end of the year. Which means if you have a FRA steam locomotive, you have to figure a potentially much higher cost per boiler operating day and/or otherwise subsidize the 1472 costs.
In the case of CATs, presuming they keep at least the 168 and 463, as well as the 315/425, that's three operable coal burning steam locomotives that need to average almost 300 days of operation a year to fully amortize their 1472 costs. Only the 463 sees enough use that in a good year (reasonable fire danger and useful for power), it might get to the 98 days per year average. However, should drought and higher fire danger conditions exist, this may restrict 463's use sufficiently to ruin the average.
Similar situation on the D&S. They might be able to justify keeping one engines as a coal burner and limit its use to after the fall rains begin until it dries out in the spring. So they could potentially get the full benefit of the 1472 on one coal burner, but maybe not two. Remember this is largely during their off season so they don't need two or three engines hot on a daily basis. Then again, in a good summer of moderate weather and fire danger they might be able to utilize coal burners more, but I have a suspicion there is now either a written or informal agreement with USFS and/or their insurer to limit operations of any coal burner.
It all comes down to $$$$. What can you afford to or not to do? At some point, it becomes either impractical or unaffordable.
