Bug: func_tracktrain "Change Velocity" methods are all broken except for "Instantaneously"

[Wheattttt]

Describe the bug

func_tracktrain entities have a KV for Change Velocity- this lets you select various velocity changing methods. However, all of the options besides Instantaneously are broken, specifically when sending a StartBackward input, to make them reverse direction.

When sending a StartBackward input: Ease in/ease out will not stop moving, and will just continue onwards, Until it hits the path_track after the one it is currently targeting. once there, it will begin jittering rapidly in place. Past the second-to-last path_track, behavior is too complex and unusual to describe in a compact manner. Watch the video for more detail. Linear blend will stop moving, but will also jitter rapidly in place, not going backwards. But if it is past the second-to-last path_track, it will stay perfectly still instead of jittering.

This same behavior happens when using other methods to reverse the direction of the func_tracktrain- StartBackward is just being used as an example.

Here is a video of the provided map. The white cube is using Ease in/ease out. The black cube is using Linear blend. The orange cube is using Instantaneously. When pressed, the button tells all func_tracktrain entities to StartForward. When released, it tells them to StartBackward.

https://github.com/StrataSource/Engine/assets/33504404/6cd42ac8-8f5b-4b1f-a0ff-d77c1379a078

Also note, that none of them actually have any kind of unique interpolation whenever they are working- they all travel at the same constant rate.

To Reproduce

1. Open provided map
2. all but the orange tracktrain will be broken

Issue Map

tracktrainissue.zip

Expected Behavior

All of the tracktrains should react functionally the same as the orange one, just with different interpolation curves.

Operating System

No response