In reviewing your particular application with "only" 6 relays there is yet another option.
I can imagine situations where it's just plain inconvenient to install a wall-wart adapter which of course requires access to AC-wall-outlet power. So let's say you really want to use the 14V-16V AC Accessory power output from your train transformer.
As discussed earlier it's that pesky bridge rectifier which stops you from simply combining DC and AC commons when the DC and AC are not isolated. So if you use so-called half-wave AC-to-DC conversion, you use just a single diode. This then goes into a generic DC-DC converter which has a common in- and out- as depicted by the dashed line. This means point A and point B are the same. But this is OK if the AC Accessory common is the same as the AC track voltage common as it will be (or can be wired to be) on train transformers.
I suggest this assuming we are only talking about a handful of relays so the 12V DC power requirement is relatively modest. The relays under discussion require about 1/2 Watt of power each. You would need to add a 25-cent capacitor as shown to provide energy storage during the half-cycle when the lone diode is not providing power (a bridge transfers power on both halves of the AC cycle). Even with the loose components (diode and capacitor) I think you might be able to do this without a soldering iron by using screw-terminals or terminal blocks or the like. If this is something you might want to pursue I can provide additional details.
