My game only works when I play it but breaks down in when someone else plays?

I used to think that this was a problem with mobile because my game would stop working when I switched to an account on my mobile phone, but now I realized that my game breaks down when on any account that isn’t my own.

So I made a roller coaster, and once it arrives to the station, guests can sit in the chairs on the carts. This worked when I tested it out. The cart stays on the station, then follows the tracks after a while, and it goes smoothly. This meant that the ride works fine, right? So I publish the game, then I test the game on my phone, under a different account, and as soon as I sit, the car begins to spaz out and fly off the tracks. I couldn’t figure out what the deal was, and made a topic here asking why the game wouldn’t work on mobile (at the time, I thought the issue was exclusive to mobile). Today, I thought I should test it again, but on my PC this time, under a different account. I wasn’t expecting this, but as soon as I sat down on the cart, it spazzed out and flew off the tracks again, meaning that this wasn’t a mobile issue, that there is something wrong with my game. I tried testing it again, but with my main account this time, and it worked just fine, the car didn’t spaz out or fly. Then I tried doing both at the same time, both my main account and my alt joined the same server, and then went to sit on the same car. On my main, it worked perfectly, then I tried to do it with my alt, and the car once again flew off the tracks.

I tried looking at any scripts inside my game, specifically in the the coaster car, that would be causing this. I use qPerfectionWeld script by Quenty for each of my coaster cars, so that it would weld. I tried looking at this script to see if there is anything there that would be causing this, but I couldn’t find anything (I’m not good at scripting, so I don’t really understand what certain parts of the script do). Does anyone familiar with this script, or with welding, know what is causing the issue? If it helps, this is how I organized my model.

Screen Shot 2020-08-16 at 10.41.13

I was also thinking that it could be related to the physics engine, but I don’t think that it explains why it doesn’t work when I was playing on PC with my alt account, because I am using the same computer that I use on my main account.

Does anyone know what causes this? Why is it that it works just fine when I’m playing it on my main account, but breaks down when someone else plays? Is the problem related to the script or something else? Thanks.