There seems to be something weird with the catenary system. Some times it does it and sometimes it doesn't, but if you try and stretch a catenary, for some reason it will sometimes "snap" back to about 1 scale metre long. The attached video shows the stretch working on one track, but having this weird bug happen on the other.

https://youtu.be/RPwOBLtPfK8

In this case you should not use the yellow square, but the arrows in the gizmo. The yellow square has a much larger snap radius than the arrows, which explains this behavior. With the arrows you can be much more precise.
 

Unfortunately the video doesn't show all the detail. I'll try using an external recorder to show it better.

Here we go.

This one has audio commentary, which is the first time I've done anything of the sort, but also it shows the widget so you can see exactly what I'm doing.  It would be even better if it showed the mouse pointer but we can't have everything.

https://youtu.be/hWsRQUckqNE

This happens when the catenary-spline is not straight, but a curve with a very high radius. It can only extend or remove a minimum fraction of a degree from that curve.
You can checkt if this is the case in the properties of the catenary.
You may be able to fix this by using the red x-handle in the gismo, so it snaps back to a straight.

Bearbeitet 15. Dezember 20213 J von Sintbert

Thanks, that makes sense.