Hi, not all sotware can be multithreading because some steps must be computed sequentially, or there are other dependences.
In the case of RailClone, some stages are multithreading (as convert the pre-computed segments to meshes), but many others not (evaluate the style workflow, pre-compute segments, etc.)
Also many functions are implemented by Max, and are not thread safe (cannot run in multiple threads simultaneously).
Really, except certain processes which are easily parallelizable (as rendering), few times you will see all CPUs at 100% working in a single task.