I was wondering why the UP put dual stacks on their FEF Northerns? I don't see the sense in doing it, it makes sense to me on the articulated engines where you have 4 cylinders like the challengers and the Big Boys but not on the Northerns. Was there any real reason as to why UP did this?

The last series FEF-3s were the only ones to have the dual stacks..................................................................................Wooly

I believe the dual stacks were to reduce the back pressure and thus improve the internal efficiency of the engine. The freer exhaust reduces tha amount of energy the engine has to expend to get the spent steam out of the cylinders.

Of course, the reduced exhaust pressure also reduces the energy available at the stack to lift the smoke above the cab; hence the smoke deflectors.

Phil pretty much hit the nail on the head: lower backpressure (= more power and better efficiency).

IIRC, an article published last year on the UP's 4-8-4's in "Classic Trains" showed an artist's rendering of the proposed FEF-4 class, which would have included Franklin poppet valves and four (!!!) stacks, for even lower backpressure.

A whole bunch of variations have been tried for steam locomotive exhaust systems. Even today, there is still some research being conducted on how to make the best-performing system.

Read more about steam locomotive exhaust design at this page on my website:

http://www.trainweb.org/tusp/exhaust.html

I must be the wrong generation.

I took one look at the subject line and thought, "OK, who's saving a bilevel container flat set already?"

*GROAN*

Punny, Sandy, Punny

Although it didn't chug the PRR. S-2 turbine carried four stacks.

Ed K. cp. Laurel Run