I've used a rectifier in the past, along with a terminal strip, to convert. I assume that I could do the same here with the eBay unit?
In principle, yes. That is, by using a rectifier at the input to the eBay DC relay you can activate with AC. But as GRJ shows in his circuit, for isolated-rail control, you might want to add a resistor (to limit current in-rush), a capacitor (to demote relay "chatter" or add some holding time delay), and an inductor (to make it compatible with DCS if using track voltage to drive the relay).
In addition to the attractive price (less than $2 shipped), a nice thing about the eBay relay modules are those screw-terminals. It can be a hassle soldering to bare relay terminals.
But the real "win" with the eBay DC relay modules occurs IF you are willing to run you signaling circuits on DC voltage (say, 12V DC from a AC-adapter or wall-wart). Then all kinds of benefits kick in. This approach has been discussed in a variety of threads but I can elaborate specific to this application if there's interest.
