Thanks for playing!
When your train gets long, switching lanes with a cart beside you will leave part of the train tail behind. This was intentional to make players think about how to slot a new train car somewhere in their existing chain. It takes a little getting used to, but you can split your train tail where you know a new car will be fit between two connected train cars and quickly snap it back together.
I didn't have time to fit that in my little tutorial, especially without making it quite long, but that's a valid point that it may be very unintuitive to do so!