Locked joints beahaving springy

I have been making a train model, and I need to make suspension for it. To make the suspension I attached one RigidBody to the main body of the train, to each bogie, to each wheel axle and to each wheelset, then I attached each wheelset to the axle by the HingeJoint, each axle to their respective bogies by the ConfigurableJoints (every movement was locked except the vertical, which was limited by a spring), and bogies to the train body by the ConfigurableJoint (constrained in the same way).

At first everything worked fine, but then I started noticing that when I tried to accelerate the model, something was causing huge drag. I figured out it was caused by that I have scaled models way too much, so I “reassembled” the same model but in normal scale in exactly the same way, and here everything broke…

For some reason, no matter what I do, one bogie stays rigid(relative to the main body), and the other one acts like a spring. It act exactly the same even if I make one bogie’s movement completely locked, or completely unlock it(set the spring to zero). It only breaks when I set it to “free”. Don’t get me wrong, I am not trying to lock them to the body, in fact, springy behaviour is quite desirable, but the issue is that the other bogie is always locked.

Oh, and the bogies sometimes switch places on restart of the editor, meaning that it is not always a particular object being locked, and another one springy.

UPD: if I increase solver iterations to smth like 100, the springy joint behaves noticeably less springy(spring becomes stronger) , but it never locks up or becomes unnoticeable(also it lags a lot), and the other bogie still would not budge even a little bit.

UPD2: Here are some pictures of the situation:

(Second picture shows what is happening on play) As you can see one bogie just would not move under any circumstances, and the other one moves like if it was a spring joint, despite both joints being configured exactly the same, and main body’s center of mass being exactly in the middle.

Okay, I solved the problem… by attaching a collider to the main body… I is really weird because it is not colliding with anything, it just rests there. My idea of what might have happened is that the center of gravity actually wasn’t in the center, and when I added the collider it changed the center of gravity to normal somehow… If you know a better explanation, be welcome to write it down!