This code is going to be retired in favor of the overlay code below.
In these examples, tiles from OpenHistoricalMap are overlaid on base tiles, which are either OpenStreetMap or MapBox Satellite. A simple slider control allows the transparency of the OHM tiles to be adjusted. The standard Leaflet layer widget is in the upper right hand corner to allow switching between base layers.
In these examples, the variations on the race circuits are drawn in different colors. The Leaflet layer widget allows switching the base layer, and also allows turning the various circuit versions to be turned on and off.
This version can fetch circuit data from both OpenStreetMap and OpenHistoricMap, which allows both historical and current circuits to be displayed in a consistent manner. See Watkins Glen and Thompson for examples of this.