2025 smoke crank and piston?

It is just a newer compatible design that came along as noted here:

The first 2025, in 1947, does not have a spring- plunger arrangement.  The crank is  configured to push against the "piston", the inner cup, and that results in the smoke coming out.  I have one of the very early locos, my first post war Lionel engine, and have had it apart a number of times, through the years.  It still runs beautifully.   The later version, which is confirmed by the drawing of the steam chest, that has a coupler molded in, has the spring loaded mechanism. The first 2025s and 675s did not have a molded coupler and the very early ones also had aluminum stacks. I believe the change occurred with an updated casting in  1949-50, perhaps even slightly earlier.

Well, that explains it.  Thanks for the confirmation!  My 2025 is the 1952 version, with the molded coupler and 2-6-4 configuration.