I’ve converted several postwar Lionel steam engines to TMCC using Electric Railroad AC Commanders, including a 671, 681, 736, 773, and 2046. They were all well used over the years by previous generations in my family, so only in “good” condition at best, and I made no changes that can’t be undone by putting the E-unit back in place of the AC Commander.
But it took me a while to figure out how to handle the whistle tenders. I ended up building the circuit described on the Electric Railroad web site here: http://www.electricrr.com/Hobby/PWW-Ctlr/PWW.htm. For all of the engines, the AC Commander is in the boiler or cab of the engine, so I ran a tether back to the tender with three wires: 1) whistle signal, 2) rear electro-coupler, and 3) TMCC antenna. Total cost of components was about $6 per whistle controller, and I built four of them. Most of the cost ($4.50) is the solid-state AC relay that you see in the picture below.
It works great! I can now blow the postwar whistles from my CAB-1 whistle button.
I mounted the circuit in the tender where the DC relay used to be. (Again, no permanent mods - I saved the DC relay and can put it back if there should ever be a reason to.)
And with a little planning of the parts layout, it was easy enough to just build the circuit on a generic 1" square piece of prototyping board, rather than go through the trouble of getting the custom PCB made as described on the ERR web site.
I've looked at that circuit and thought I might give it a try one of these days.