mini fuse?

The ATC blade type are fast enough. The poly-fuse would be ok , also. I don't think it's the amperage, but rather it's the voltage spike that does the damage.

What GRJ recommends is a TVS inline with the power feed from the track power collector to absorb the voltage spike, wherever it comes from.

That's my take on it.

If you want to add protection to the locomotive, IMO the TVS is a worthwhile investment, one in each engine, and for Lionel IR tethers, one for the tender as well.

A fuse isn't going to stop a voltage spike, which is what usually does the damage when electronics fails and there's no apparent cause like shorted wiring, etc.

 I guess that what I was after, was protection from those times when a wire did get shorted like when they rub on the frame or get squashed when the shell is installed. I used to install a glass in-line fuse in other scales when I added DCC. I have wandered away from that in O scale DCS installs and think I should go back to it.

 I use the TIUs in active mode even on my large G scale layouts. I'm hoping the TVS inside will do the job. Maybe I should add a TVS and a fuse in every install????

The absolute best place for TVS protection is closest to the thing you want to protect.  While the TVS in the TIU is good, stuff that happens well away from the TIU can generate a spike that gets to the electronics of the engine.  The impedance of the track connection to the TIU for a high energy, high frequency spike is a lot more than the DC or 60hz AC impedance.  The farther away the TVS is from the protected component, the less likely it is to provide effective protection.