I've used the plaster method before. It takes a little patience, but looks good. Spread the plaster even with the tops of the rails using the widest putty knife that you can. Completely cover the track - both inside and outside the rails. Don't wory about keeping the flangeways clear. After the plaster has set up, you can carve out the flangeways later using a tool made from a sharpened nail. This is the basic method that was described in a model railroader project layout back in the early 90s. The port of los angeles layout, I think.
Kevin