It was asked and "not really" answered THEN:
http://cs.trains.com/ctt/f/95/t/8812.aspx
I have no personal experience, but would go out on a limb and offer some thoughts:
1. The transformer will probably absorb any arcing, and not pass it back through its windings into the primary.
2. Since AFCI breakers look for arcing between the hot side and either the ground or neutral, and since in Post-War toy trainland, the wires going to the track are not grounded nor are they neutral, (in other words, they are "floating") I would say there will be no problem.
The 2008 NEC requires the installation of combination-type AFCI's in all 15 and 20 ampere residential circuits with the exception of laundries, kitchens, bathrooms, garages and unfinished basements. So, if your layout is in one of those areas, you technically don't need an AFCI.
3. I would imagine that AFCI might be more apt to get confused and open by mistake if they were to encounter the RF signals used in modern command control systems. But this answer is REALLY based on conjecture, with absolutely no guaranty that I'm not talking through my hat.
