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This free 1g of turning force is apparently known among pilots as “God’s G”. It needn’t be produced as lift while you’re inverted at the top of a loop, so you get a good turn rate despite losing speed and hence lift as you get up there.

Not just gravity, but the train also loses speed at the top, so the loop needs to tighten if you want to maintain the same force (you might not want this).

Also by the time you get to the apex it’s going slower than when it entered the loop.

I’m having trouble imagining a scenario where this is not the case.

Here's a 1 reasonable and 1 contrived example:

An inverted loop, where the loop is lower than the track, and with a very long train, where the center of mass continues to decrease in height due to the part of the train not presently looping.

Roller coaster cars with electrical motors? I somehow like the idea. Could mess with Gs in one more direction.