EPA has been on steam locomotives for about three years, hired the firm I work for to help gather data on the matter. Proposed rules should show up next year. EPA will likely ban use of waste oil and likely to require use of ultra low sulfer diesel if steam is permitted
at all. Several issues at play here including the lack of exhaust emission control. Change to ultra low diesel will pose some significant challenges as that blend is not too far from being gasoline at this point (as anyone who has had to deal with it in older diesel engines knows.)