Upgrade Lionel 6-1587 B&O Capitol Limited Set with 8304 4-4-2 Steam Engine and Baby Madison 9500 Series Cars with dummy Couplers

I've pondered this type of conversion myself.  You would be correct to use only one of the two field windings.  The Dallee 400 is solid, but as you concluded, it's too big to fit in the loco.

A second option is to search popular auction sites for the out-of-production QSI ACRU-E, which has a square-ish form factor, and was designed to fit in a steam loco, in place of a 3-position mechanical E-unit.  There's a hitch: your stock 2-position E-unit has no plunger, so it's SMALLER than a 3-position model.  Even with the ACRU-E, it might be necessary to cut away the rounded part at the rear of the roller pickup bracket to make it fit.  [The only MPC-era four-wheel loco sold with a 3-position E-unit was the 8506 switcher.  To my eyes, it looks as if the plastic roller frame on these was cut at the Lionel factory.  If you own one of these switchers, or can find an example for close study, you'll see what I mean.]

Re: Sound of Steam, it's true that the capacitors may dry out but they can be replaced.  Early versions of this board used only discrete solid-state components (no chips), so the whole board is repairable.  Personally I like the Sound of Steam so I would be inclined to retain it.  If you want to add Whistle, the easiest way to do so would be to replace the whole board in your tender with a combined Sound of Steam / Whistle board sourced from an 8204, 8206, or 8305 loco tender.  You'll also have to add a roller pickup to one tender truck.  [NOTE: your KW may NOT reliably trigger the early MPC whistle, as these locos came with their own (and unfortunately failure-prone) whistle button that probably has a different DC offset. ]

I don't want to discourage you, but my parting thought is "temper your expectations."  I've owned a handful of these MPC-era 4-4-2s.  Only one is a decent runner.  They seem prone to overheat, slow down, and stall, refusing to budge until after they cool down.  Some folks on this Forum suggested replacing the brush springs or increasing the spring tension.  Before receiving that advice, I was planning to swap the MPC-era driving wheels with their side rod bosses onto a postwar metal "Scout" motor (part number 250-100.)  The 250 motors came from the factory with a (mechanical) 3-position E-unit.  It's a bolt-in replacement to the 8304 with all of the same mounting points.  IMO the postwar motor runs better and looks more realistic with its black steel side frames, even if it's less powerful.

Good luck whatever you decide.  Please post back and share some photos of your progress!

Thanks, Ted, for sharing your valuable experience, comments, and suggestions. Especially related to the MPC-era 4-4-2 motors (i.e., they are prone to overheat, slow down, and stall). If I am to invest in this project, swapping to a post war 250-100 motor is a valuable suggestion.

Although I ruled out ERR and Command Control in this Post, I have since revisited the ERR website and found I can retain Sound of Steam, Chuff, and Whistle with ERR components plus motor control in a small footprint (e.g., Tender).

(I have already successfully converted one MPC era Diesel to Command Control using ERR components when ERR was still part of Lionel.)

As a side note, regarding the KW transformer and DC offset, you may be interested in my recent Post on this topic - Lionel KW Transformer Service Manual Errors/Corrections using the link below.

https://ogrforum.com/topic/lio...l-errors-corrections

Yes, an ERR AC Commander with RailSounds would be the "nuclear option!"  And there's no issue with the antenna because the command signal easily penetrates the plastic tender shell.

Disclaimer: I've not done a 250-100 motor swap on a 4-4-2 (although I have no doubt that it's possible.)  I have done it on an MPC-era 8042, which is a 2-4-2.  The key difference is that  the "Columbia" or Scout-type locos typically had only one main rod to the rear driver.  I re-used the original main rod and hex screw from the 250, the rod mates right up to the MPC-era crosshead.  If you want to keep the nice tandem rods from your 8304, you'll have to pull the MPC driving wheels, and re-press them onto the axles of the postwar motor.  Pulling and pressing wheels is an advanced repair best left to experienced repair persons with special tools.  Quartering isn't too critical because the large spur gear in the middle drives both wheels.  However, it's easy to bend an axle or press the wheels on crooked, which will make the loco wobble and hop.

The last attempt I made at a transplant before I tore down my layout, was to start with a 1615 postwar switcher motor.  These have four wheels that are already quartered and tapped for side rods.  However the hex bolts used on the 1615 are a smaller diameter and not directly compatible with the drive rods used on the 8304.  With a longer bolt and a piece of brass tubing or a spacer it might work, but the fit has to be snug because the right-side rod works the smoke piston and contacts for the Sound of Steam.

Every time a new catalog comes out, I hope Lionel will reissue something like a postwar scout with a high-performance LionChief Plus mechanism that could be used to repower our old classics.  In a recent catalog they brought back the Pennsylvania-type "675", so I guess there's hope!!