Hi, Brian,

Maybe red and brown wires swapped in the connection of the main track?
CS1 gives the same power to main track and programming track unless programming mode is active.

Brian, just check the address & see if there are any other locos with same address. Could be a conflict but it should not occur. I would suggest you remove that loco from the CS1 memory bank & start all over again & if any other loco with similar address just change your one manually on the loco to an address that is unoccupied. Give it a try & see what happens. Your 6090 decoder is using Motorola format. Tick the correct boxes. Also remember you only need one address for that loco & sniffer address should not be necessary & you will have an 8 function selection. It is a while ago since I used one & I may have it all wrong. Good luck.

Hi!
The batteries are only needed when you turn off the power without shutting down the CS1 previously.
If you always shut down the CS1, no batteries are needed. Leaking batteries can damage the CS1. I never had batteries in mine.

As mentioned previously, the programming track output is normally connected via a relay to the main track output. If the loco works on programming track but not on main track, then I suspect there is a hardware problem related to the main track (swapping red and brown is the easiest explanation but you said that was not the case here (but triple checking cannot do harm)).

Test the loco on a short piece of track connect to the main track output. If it works there, but not on the layout, then search the problem on the layout and not in the loco.
Is there a 74046 used on the layout? Must be removed for digital operation. Maybe there is something else that disturbs the digital signal.

Are other locos with 6090 decoder running fine?