Here is a bridge and capacitor mounted on a barrier strip to provide DC to an individual relay. If a separate transformer is used,only one is needed for all the DC relays on the layout. This one is good for 4 amps,enough for 80 watts of 24 VDC relays. Relays pull from about a half watt to 2 watts,depending on design.
Shown is a DPDT relay,more useful than you may think. The second set of contacts can be used for example to switch a block signal on the siding,an indication if it is powered or not.
Dale H