When placing a blueprint of, e.g., a single wagon, next to an existing (non-ghost) wagon or locomotive, the ghost position "snaps" to the existing train, just as it does when building a train normally. When the ghost is placed, however, it is placed at the exact current mouse position rather than the snapped position.
Not sure if this is the intended behaviour or not, but seems like it might be a bug so I made a post.
[0.17.4] Train blueprint snap to wagon
Re: [0.17.4] Train blueprint snap to wagon
This was most likely solved with the solution for 68147
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