In line fuse and/or circuit breaker?

I would use both plus a TVS. Automotive fuses are fine. 10 amp I would not use anything bigger than 10 amp. I'm sure others will chime in. TVS's are a must.

The TIU has a TVS on every output channel, so you don't need to add them there.  It wouldn't hurt to add them farther down the line.

On the forum the advice at one time last year was to add a TVS on each set of posts.  So I did.  Now it's "not necessary??"

It has since been pointed out MTH added them in the tui.

Leaving them there, or moving them elsewhere around the track, like near a finicky turnout, isnt going to hurt.

   You likely aren't testing them regular for a bad one anyhow, so each one is  additional protectection. If I recall correctly, after X amount of surge, they are "used up".

Actually, it's the MOV that gives up a little of it's life for each surge.  Other than random semi-conductor failure, a TVS won't die unless it's overloaded by a really large transient that exceeds it's ratings.

In answer to your other question, the voltage rating doesn't matter on a train layout.  The rating is the maximum voltage that the fuse or breaker can open safely.  Only amp rating matters, since an amp is an amp (a fixed number of electrons per a specific time period*) no matter what the voltage.  I would not use slo-blo fuses.

*From www.dictionary.com: the basic unit of electrical current in the International System of Units (SI), equivalent to one coulomb per second, formally defined to be the constant current which if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed one meter apart in vacuum, would produce between these conductors a force equal to 2 × 10 ⁻⁷newton per meter of length.