Two-conductor Output cable from Lionel 72-watt Wall Wort - which wire is GROUND?

Its AC, it doesn't matter.  If you are using more than one transformer, then you need to phase the output.

Ops, that's a DC power pack.  You'll have to test the output with a multimeter to determine what wire is negative.  Just a guess, but the outer barrel of the plug is negative. 

Put the probes of a multimeter on each lead. Set it to DC, and if you get a negative reading reverse the probes and that's your negative and positive

Usually the one with a white stripe or ridge is the hot + wire on a DC pack. Use a multi-meter to confirm. If it goes negative, you have it reversed. It does matter for AC too, but only for polarity and phasing.

George

George S. and others ...

Thanks for your prompt and helpful comments.  I'll apply your suggestions today when I solder-on the "Size L" coaxial plug to the output wires.

With appreciation,
Mike Mottler    LCCA 12394