I'm trying to remember what resistance the smoke unit element is, and seem to remember it being something like 27 ohms. I'm not certain that full track power is run across the resistor here, but even if it is the heating resistor would only draw about 2/3 of an amp.
So, in terms of transformer load, at 18 VAC, you're looking at like 12 watts of consumption. in the end, a CW40 is enough to run a single tmcc engine with smoke under normal conditions. Anything past a starter set transformer is more than enough.
JGL