I agree, it is Roblox, but I was about to reproduce this myself again today by ramming my train into the back of someone else and it caused my train to start to fall apart and the parts continued to float out in space and move along with me when I was driving my train. In this case, it was the doors on the side and my train was still moving until it finally flipped off the track because the doors got “caught” in something out in space. 
A few questions to help with understanding this:
- When this bug happens, does the missing part of the train look missing for everyone or just a single player? (Trying to rule out “Local Simulation vs Authoritative Simulation” desync)
- Is there a freeze/long frame when the train car disappears?
I’m not the OP, but I can answer question 2 for you:
In video 2, you can see the statistics on the top left, where the FPS drops before it stabilized back to consistent FPS during the dropping of the train car.
Now I’m lost on what you’re even doing… 
- Yes, the train car becomes missing for everyone.
- I believe so, though I cannot confirm and it may be the case that it only freezes for some/sometimes
Someone managed to get a good shot of the moment it occurs!
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So, are there any fixes or workarounds available to combat this? Because guess what? I have the same issue, but even worse: When the player sits in VehicleSeat, train cars/coaches can dissapear later on with a rare chance if the client is PC, BUT if the client is Mobile, then a truck/bogey/bogie would dissapear with a high chance!
Ive combatted this partially with NetworkOwnership script, but i am not sure if the issue still persists…
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This is getting worse: now for some reason my players can’t jump from the VehicleSeat. From server side view they look like they are stuck in the ceiling.
We’re no longer getting any reports of this issue in our games. However, another game is now experiencing it: Project: Laurelton - Roblox
External Media External Media
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Is it because of the different physics settings located in Workspace instance? Ive heard of PhysicsStepping setting or something…
Not sure, but we didn’t make any physics changes on our end.
Try inserting a script that constantly prints information about every car’s position, rotation, vectors, etc. Maybe it would glitch out, and it would show it when the car dissapears.
Something like this, in situation where train is decelerating (oversimplified version)
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8957324948092384248097534095893847589
nan
(Car № has dissapeared)
For some reason, this bug disappeared for a bit, and then resurfaced yet again. It keeps appearing and disappearing. But the fact is that it’s currently happening, and that’s a problem.
Screenshots from another game:
It’s wonderful how we have an issue that disappears for a few weeks only for it to suddenly come back again then it disappears again and repeat.
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I guess that looks like a defect of the servers (?): I think the enginirs actually fixed the issue, but they deployed the fix only to some servers… Or it just depends on something: the age of the server maybe?
I am sending this using a translator, so there are some things that are not right. My apologies.
I have a workaround for this problem that I can’t guarantee, but I have a band-aid solution that I will share with you all.
Briefly, I have been able to work around the problem by “Weld before it is anchored and then remove Weld after it is unanchored”.
With the exception of WeldContraint, the other -Constraints will not behave as they should if extreme force is applied, so it is very likely that this bug was probably also caused by extreme force on the car body.
I hope this is a workaround for this bug.
I’m not sure what you mean by this. Could you lay out the steps?
When anchoring,
- Weld the parts to be anchored and the parts that are connected by hinges but not anchored.
- Set the anchor of the part to be anchored to true.
When unanchoring
A. Release the anchor of the anchored part.
B. Release the weld applied in 1.
This way I was able to avoid this bug.
Sorry, I still don’t understand…
The way our trains work is that they are originally anchored as a group model. They are inserted from the group inventory (still anchored), and then unanchored.