The last time this discussion came up, I put a Fastrack switch on the bench and went looking for voltage transients with a 100 MHz Tektronix scope. If you hit it just right, the switch will occasionally put about 30 volts on the center rail, but it is very narrow and doesn't happen often. Having said that, I have not checked to see what happens in real layout conditions; Pullmor motors and all that. But, I feel confident that the switches themselves are not creating any damaging transients.
FWIW, it is very easy to underestimate the amount of electrical noise created by PW locos and accessories. Some time ago I was repairing a 364 log loader, and wanted to see if DC operation would quiet it. Had a Heathkit battery eliminator, so used that. To make a long story short, the battery eliminator failed repeatedly. Turns out that the 364 motor was generating happy little transients of about 150 volts, which wiped out the regulator chip in the Heathkit. Now, Heathkit was not known for fine designs, but it serves to illustrate what PW motor and accessory designs can do to electrical systems.
Since that time my track power comes from a dedicated transformer, nothing on it except the center rail.
