IIRC the Mainline Manor has a white plastic pod mounted motor with metal contact strips across it from a bolt to the metal casing of the brush holder. You need to break those contacts, possibly the easiest would be piece of plastic card on the brush end of the metal strips with further metal strips on the brush side of the plastic card which the motor wires from a decoder are soldered to.
The Bachmann ones are more difficult needing a virtually complete strip down off the chassis as shown in this Youtube video
https://www.youtube.com/watch?v=BY5Qitms23ofor the Royal Scot. A multimeter is essential to check when reassembled the motor leads are still insulated from the pick ups.