I have Bachman manuel G scale switches and would like to throw them electrically.  What type of switch machine and what power do I need?

I want to be able to use a pair for a run-around siding so that when the switches are thrown, the Main line is dead but the siding is hot.  How is this wired?

I am using all Bachman track for an around the room shelf layout, so manuel switching would be difficult.

Thanks.