This usually means the shape has a corrupt sub-object count. Run the "Repair Sub-Objects" tool. If that fails, find a clean backup of the shape.
MSTS Shape File Manager (SFM) 2.5 is a Windows utility for Microsoft Train Simulator (MSTS) that helps modders view, extract, edit, and manage shape (.S) files and related asset packages used by MSTS. It is aimed at route and scenario creators who need a convenient way to inspect geometry, textures, and metadata stored within MSTS shape and package files. msts shape file manager 2.5