Train Spline controller?

Hi,

I am making a sort of train simulator but am having trouble perfecting a multi-car train spline controller. I am interested in replicating what I see in this video. Can anyone glean how they might be approaching this controller?

The track system I have in place is modular and prefab-based, unlike in the video, as random path generation is something i'd like to implement in the future. Each of my tracks has it's own spline aligned to the model and the meta-spline path is calculated by a pathfinding algorithm on start() with each track ordered and added to a list to be referenced by trains as they move along the path.

I currently handle train movement along the path via an iTween implementation where each track has a normalized beginning and end range(0-1) called progress.

     progress += speed / currentTrack.length;

I assign a train's progress via copying the train in front of it's progress and subtract an ideal train distance that has been arc-length parameterized for consistency.

     float GetNormalizedProgress()
     {
         //progress = train in front of me's progress(-)the normalized train-to-train distance
         return GetTrainByIndex(myTrainIndex - 1).progress - 
             train_to_train_Distance/train.currentTrack.length;
     }

When a train reaches 0 or 1 progress, the current spline index is set to the previous or next spline in the list.

 //returns the correct next track
         int GetNextTrack()
         {
             if(progress > 0 && progress < 1) return train.currentTrack;
             if(progress > 1)
             {
                 progress -= 1;
     
                 //if there is a track after this in the array...
                 if(train.currentTrack == 0)
                 {
                     return tm.orderedTracks.Count - 1;
                 }
                 //else, keep going forward
                 else
                 {
                     return train.currentTrack - 1;
                 }
             }
             else if(progress < 0)
             {
                 progress += 1;
     
                 //track has reached the end, set to beginning
                 if(train.currentTrack == tm.orderedTracks.Count - 1)
                 {
                     return 0;
                 }
                 else
                 {
                     return train.currentTrack + 1;
                 }
             }
             return train.currentTrack;
         }

This implementation has worked OK under some circumstances but often produces very glitchy results for heretofore unknown reasons. If anyone has some insight on how, in the video or elsewhere, the train controller could work, or how I might approach it within my described modular system, I would appreciate it greatly.

Thanks!