I did exactly what you are saying and it works very well for me. I used 3 - 3 pole contactors that activate from 12 volts which is the only thing live in my train room prior to firing the contactors. I use a key switch on the control panel to have the ability to remove the key if needed and also I had several in the ol parts bin so the cost was nothing.
As long as the contact rating on the relays are enough for your load, you are good to go. JP
