Chromatic Homotopy Theory
《Chromatic Homotopy Theory》 · 第 246–247 页 · Section 8.3.8，Train tracks 的完整定义及 Figure 8.5 原图
A train track in a closed surface \(S_g\) is a closed subset \(\tau\subset S_g\) built by taking a finite set of points (called vertices or switches) and joining them with disjoint arcs called branches. We require that every switch looks locally like Figure 8.5-(1): all branches have the same tangent line, and there is at least one branch at each side. The complementary regions of \(\tau\) are the abstract closures of the connected components of \(S_g\setminus\tau\). Each complementary region is naturally a compact surface with finitely many singular vertices in its boundary having “zero interior angle”. In the definition of a train track, we also require that no complementary region be a disc, an annulus, a monogon, or a bigon, as in Figure 8.5.