Odd Problem: place, before the extended area of track where a train just stops


Appreciative Trainzer
Over a period of time I have been extending a route.
I placed a new baseboard and extended lengths of 2 x tracks.
Now something strange happens when running any train - in either direction.
There is a place, before the extended area of track where a train just stops.
The speedometer does not return to zero. The train cannot be moved. Smoke still comes out of the engine.
I have tried a few things to try and work out what's going on - including lengths of the same track 30 metres either side of the double track.
Same thing happens.
The rest of the route is fine.

I am running the latest version of Trains+.
Any ideas of what is going on?
I saw the same behavior in Trainz Plus. In my case, it was using a pre-fab yard and the curves through some turnouts were apparently too sharp for the loco being used. The loco would just stop moving at random locations but would continue to produce smoke and engine sounds as if it was running at speed. Using a loco with a shorter wheelbase or increasing the radius of the curves of the turnouts removed the issue. Odd thing was the turnouts looked fine and no sharper than turnouts that I have used for 20 years.