What I have done to several older Lionel F-3 sets that I like which haven't been made again (Illinois Central City of New Orleans and Santa Fe blue freight scheme) has been to buy later version F-3 sets that were already equipped with TMCC, RailSounds, electrocouplers, directional and constant voltage lighting, finished interior, etc., and swap the shells. I then sell the shells from the newer engines on the Bay.
This may be somewhat more expensive than going the ERR route, although adding ERR TMCC, RailSounds, electrocouplers etc. plus installation to multiple engines can get expensive. In any case, I end up with TMCC etc. not only on the powered engine, but on the dummy A and B as well, so all dummy engines are fully equipped with remote TMCC electrocoupler, directional lighting and constant voltage lighting,in addition to the RailSounds in one of them.
I also take the finished cab interior with crew figures out of the newer cabs and put those in the other shells, and mount aluminum foil inside to reduce the cab lighting to a low level. Another easy improvement: use two or three layers of masking tape and tape those inside the cab over the cab number lenses - it dulls those down nicely, and makes the numbers readable as well. Anyway, the end result is a nicely upgraded set of engines.
The only part of this procedure that takes a little doing is swapping the RailSounds speaker. The F-3s that come with RailSounds have a molded-in circular frame with screw receptacles that the speaker mounts to on the roof, and that has to be replicated. On one of the sets, I just cut out the top of the shell that had the speaker frame and glued into the inside of the other shell. That destroyed the shell, of course, but it was fairly quick to do. On the other set, I found a cap from a jug of laundry soap that was exactly the same size as the speaker (2" across), and cut that out to fit inside the roof, glued that in, made four small posts for the mounting screws to screw into, and went with that. That procedure took a fair amount longer (including time finding a workable circular frame), but it worked fine. Of course, the second time doing something like this would go far quicker!
I don't know if the ERR RailSounds upgrades for traditional F-3s come with a roof mounting frame or not, but I doubt it, and am not sure how the speaker would be mounted. Note that the speaker openings, which are open vents in the roofs, are already in place on all the F-3s, which allow the speaker sound to vent the shell. On the newer engines and some earlier Postwar, there are screens covering the openings, and the other F-3s have louvered vents. The speaker is positioned directly under the vents.
Sorry for the long-winded explanation.