To my knowledge there is no easy way to do this unless the creator specifies it in his description. If you have TranzObjectz and TRS2004, TO will give you an estimate of the "poly" count based on file size. As I understand it, all chunky mesh track have the same poly count, that is, 2 polys, since it is based on a flat plane mesh and a string of numbers in the config file which tells Trainz how to shape the plane in the game. To assess which chunky mesh track is higher or lower in "poly" count you would need to look at the number and file size of the associated TGA and BMP files...the fewer and smaller the better for performance, if not for appearance.
Modelled track, if it follows the profile suggested by Auran in the CCG, is going to have a minimum poly count per segment of about 18 to 20 polys in the mesh. Once again I think, the number and size of the graphic files will probably determine performance. For modelled track, using LOD techniques can help performance.
From using my own high poly MBTA track, I would say that for a single track or double track line, high or low poly track doesn't have much of an impact as it is the totality of all the stuff you put into the scene that determines frame rates. This would not be true for large yards and such however, where low poly track will make a real difference in performance.
Testing things out, as suggested above, is always useful.