The relay modules that I've found are rated at 10 amps at 125 VAC. With the low voltage AC used to run trains, these are perfectly suitable to supply power to the rails; especially when you have a layout divided into different blocks.
The diodes on the relay boards connected to the coils protect against noise from back EMF (electromagnetic force) which occurs when a relay is turned off and the magnetic field of the coil collapses. That will at least help with the electronics on the board itself. But it will not affect the contact side of the relay outputs.
A TVS is always a good idea to prevent voltage spikes on the tracks when you are running modern (electronics based) trains. The relay outputs from one of these modules might be a good spot for those. But you said you're running a conventional layout. TVS diodes won't help you much there.
That buck converter looks very handy for powering the relay boards. Could I also use a second buck converter (or some other DC supply) to "power" the isolated track sections rather than connecting the isolated rail to my AC accessory terminal and then using a bridge rectifier for each relay input? I guess the real question is can the DC negative output of the buck converter be "phased" with my transformer by connecting the negative output to common, since that would have to be the return path though the non-isolated outside rail.
I'm not sure what to say when it comes to mixing AC and DC conductors. I try to avoid it myself just because I don't understand it.
You could feed the AC ground from the isolated rail into the inputs of the buck converter to power it up from AC aux power. That would give you a DC output as inputs to the relay module. But there are easier ways to achieve that.
I would want to use a bridge rectifier with a capacitor and maybe a voltage regulator to produce the inputs to the relay module.
The inputs to the relay module require very little current. I've run them straight off an Arduino which is about 20ma.
