How do I link a model with a chassis?

Hey everyone,
I have just created a chassis for my train. I already have a pre-existing model that I want to link with the chassis, so I can drive the train. I don’t know how to. I have already tried to add weld constraints so that I can link the parts of the train model with the chassis part itself (the part the wheels are connected to, via a hinge.)

Is there a way I can link the train model with my chassis I created? Thanks!

If you need me to get into more detail, I can.

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Here is what my chassis looks like with labels for each part.