So the fixed lines *could* power the fixed output to the siding, but the result would be having to lift any PS1 engine off the tracks in order to move it to the layout's main lines... Possible I suppose, but not ideal.
I guess an alternative would be connecting one of the variable outputs on the Z4K to a variable port *and* a fixed port on the TIU. The variable line connecting to one of the main loops, and the fixed line going to the siding. That would let me vary voltage to the main line with the remote, and on the rare times I need to drive an engine in or out of the siding, I can use the transformer's handles.
As a separate question, can I power the TIU by connecting the fixed line 1 to one of the fixed outputs on the Z4K? Or is it better to use a 'jumper cable' from the variable port which is being powered by one of the variable outputs from the transformer?
As for the BCR, ya that's something I've already looked into. Not sure if it's worth it though since I plan to upgrade my last 2 PS1 engines to PS3, which will remove the need for a battery anyway.