Its strange that Lionel didn't post the manual for this one on the website.
The parts directory shows a dual smoke unit under the stack, so that would indicate filling through the stack.
Keep in mind these dual units are thirsty and you may want to give it a "night before" fill , or wait a day to try the smoke unit so the fluid has time to get absorbed into the second reservior.
The fluid only enters one side through the stack, than it gets absorbed into the other side of the unit once it gets under the divider.
