There is a chart on the board. We recommend that as player's complete them, they place one a locomotive marker of their color on the chart to show it has been completed.
It is also suggested to use some matching markers (like glass beads) to mark the start and end of each Major Route, to make them easier for players to see. Use a different color set of markers for each route, of course.
Just to clarify Ralph's mention about using matching markers for the Major Routes, you'll need a set of three in each color for each Major Line. One to put on the major line on the chart itself and a pair for each end of the route.