If I try to copy a train, it won't begin until there are sufficient parts available, including fuel. However, some of that fuel is loaded into the temporary engine used to move the cars during construction. That causes the construction to fail when an engine can't be fueled.

This creates a destructive loop where there isn't sufficient fuel, the train gets partially built, and then gets destroyed along with what fuel was loaded. That makes it all the more likely that the next attempted build will fail for the same reason.

Note that trains destroyed during a failed build process don't recover the inserted fuel (or the engines and cars either, I think). Even on a successful build, the fuel in the temporary engine is not recovered causing the loss of a full load of fuel every time with every build.