Thinking a bit more about the topic title, there's another consideration for using 12V DC relays for block control. Once you have 12V DC available, you can apply low-cost eBay ($1 to $2, free-shipping from Asia) timer modules to add interest to block control operation.
For example, the Lionel #253 timed block signal, #132 station stop, etc. are/were fun accessories that, for example, could stop a train for 15 seconds (or whatever) at a station and then re-apply track power to send it on its way. The timing was based on a thermostatic bi-metallic switch which would heat up from AC track power heating a resistive wire.
Here's an OGR thread using 12V DC eBay timer modules, in conjunction with 12V DC relays, to control block power in a sequenced trolley station-stop. Here's the wiring diagram I suggested. Note how I used an isolated 12V DC wall-wart. But now that I've fiddled with the half-wave DC-DC converter, I see I could have also used AC Accessory power as provide the 12V DC with a common AC and DC to allow insulated rail triggering!
The wiring diagram is a bit tedious, but it also uses two of the four relays to implement a latching relay function which can be useful in block power control. Yes, there are latching AC and DC relays but are typically expensive. So here we're talking applications where an insulated rail trigger could be momentary (as the train passes by) but you want this trigger to "set" the latching relay to continuously apply block power to some section of track until another insulated rail trigger "resets" the latching relay.
