These days engines are ever more clean and lean, while their exhaust gases are being scrubbed down to remove unwanted particles, and reduce their carbon footprint.
The latter of the two is threatening the economics of actually building DE locomotives, and the EU directives are being fought apparently. The contention is that "scrubbing" and majority-removal of CO2 in locomotives requires such a large volume of space and weight that they squeeze out the traditional low emission "mid speed" marine derived units ( Read CSVT / RK 270/ 215) in favour of the hotter, lower service interval higher speed ( in fact "mid speed too" ) of above 1100rpm.
The americans are going back to the future, by adding a tender to locomotive prototypes in the aim of having Zero emissions.
In all this scrubbing and need for a small, super powered engine ( ie Power Unit in railway talk) there is a little hope for the most wonderful PU of them all; the Napier (rustons) Deltic.
The engine was horribly smokey because of its two stroke, swept port format and rather low compression for a two stroke not helped by primitive "blowing" in the non turbo version.
However, if you need to scrub it all out anyway, then in fact all this "clag" ( a good deal of which is actually steam anyway) is not such a hindrance. In amonia CO2 recovery, the solutuion is usually recycled many times until the load of carbonates is high enough to render further use of the solution inefficient.
Other things could be done to the deltic unit to make it leaner and cleaner:
1) use of ceramics to replace high tolerance piston rings and other components.
2) Common rail to reduce fuel use and inefficient combustion on run down and idle
3) Rotating cam actuated valve gear for the exhuast port: meaning the port is closed as the oiled section behind the final piston ring sweeps past
4) high pressure super charging with intercooling : used prior to full turbo engagement, this would increase the manifold pressure and apparent compression and reduce inefficient low rpm combustion
If we track the advances in the valenta-ventura-VP series of engines, then we could perhaps expect a specific power increase per cylinder enabling a four bank, 12 cylinder deltic blown by both super-and turbo-charging to produce in excess of 2000hp/ 1.4kw.