Re: 491

Here's a pretty good article that was done by the Coalition for Sustainable Rail a few years back on the MMFE. In its most basic form, it's a series of baffles and screens in the smokebox that helps break up cinders and better channel the flue gasses for a more efficient draft.

Article

Most (if not all, someone a little more knowledgeable here feel free to chime in) of the D&RGW's narrow gauge engines were built with these, and many of the smaller engines retained them until the very end (346 still has hers, and 20 will have one). Sometime in the early 40s, pretty much all of the bigger Ks (again, not sure on the K-27s) had theirs replaced by cyclone front ends. These proved to be better at spark arresting, and used the draft to help clean themselves out- a handy feature, considering that it's a messy, tedious thing to clean out a MMFE. The easiest way to spot the change on Rio Grande engines is to note the spark arrestor, as after the advent of the cyclone front end, very seldom were arrestors used.