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.