An old Lionel O27 manually operated switch can be powered by a car door lock motor/actuator. The photos tell the story.
The switch's original lever and geared cam are removed, the metal tab is bent upward perpendicular to the switch deck, and the door lock motor/actuator is glued to a piece of styrene attached to the plastic switch deck cover with a glue designed for use on ABS and styrene.
The tab is attached with light gauge wire to the actuator plunger tip. The motor/actuator is a 12 VDC motor, but it works well on 9 VDC, and is wired like a conventional HO scale turnout. It is not non-derailing but the switch's internal electrical rail contacts can be used to route current to the adjoining blocks, preventing most derailments. This motor/actuator is a surplus item from www.AllElectronics.com.
The motor/actuator can be placed under the layout with a little tinkering.