I'm right in the middle of building my new layout, Eastwood Town, and have never built platforms before. I've read various bits and pieces on the web and they talk about laying the track and then marking the edge of the platform by using a pencil held on the mid point of a coach to mark out where additional clearance is needed for the overhang.   That's OK, but I would prefer to fix the platform edges in position before laying the track as they will act as barriers when it comes to ballastin