you are right the software needs a backround, but you make one mistake here, you have to rotate the hole background with your object. 
so far i have understood, you make a pass with your cam on your lego train, then you rotate your object. in the next pass you are capturing the rotated object, but the feature rich background is at each pass the same, thats the problem.
Did you try to import only one pass of images, without rotating the object? i think then the images should align correctly.
And the software needs features to detect, the software makes no decision what is a background or an object you want to capture. So no background, no features, no problems
Also try this settings: https://support.capturingreality.com/hc/en-us/community/posts/115000773791/comments/115000547791