When you move the train, do all the Parts move with it, or are some attached to each other, but not the train chassis?
Also, where are the Attachments in your highlighted Part?
Green Welds are active, between 2 Unanchored Parts.
Grey Welds are inactive, between 1 or more Anchored Parts.
If you mouse over the Weld you can click the Alt key to get more information from the connection.
You may want to see what’s up with that.
I usually have one large Part as the ‘base’ for my vehicles, and Weld each of the other Parts to it so I don’t double weld Parts together. I select the base Part first, click the Weld Constraint tool in the top toolbar, then select the other Part I want welded. This way I don’t get the doubled up welds you appear to have.