From my experience I have found the following causes of stutters and lockups.
An old Ground Texture.
An old Wire mesh fence.
A 1930's car, fixed by deleting and re-downloading it.
A Speed Tree caused crashes.
A few SpeedTrees are FPS killers even in TRS19.
A very high poly magnolia tree, not a SpeedTree.
A junction that went downwards below the base board, didn't have a lever so wasn't obvious until I went to wire frame, where it was going downwards but in line with the main track, not sure how that happened.
Very high poly people, clearly 3ds max or similar models made for static use not games imported to Trainz with no Lod and used in large numbers.
Forget which ones, but models that had a massive bounding box for a small item, probably a stray vertice or vertices not noticed when exporting, those can be picked up for im meshes using Pev's mesh viewer, they look very tiny and the detail show a ridiculous size. I'm pretty sure TRS19 now picks these up as a warning now.