Another factor to keep in mind in the block signal- if using insulated rail, then in theory, the relay coil is not activated on for long periods- think hours or days of constant ON state. If using conventional operation, track or accessory voltage you are again in that 8 to maybe 16V range of AC, so that's within reason in the 12V specification given the expected duty cycle.
The only scenario that might be a problem is a train parked constantly for a long time activating the coil, and the coil was seeing say a full 18V AC. Then you might get into that overheating- but using an accessory voltage or other lower voltage would allow near unlimited operation time without any worry.
