With these bookends it looks like a train is on its way through a tunnel in your books.
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Nice, but how heavy are they?  Did you add weights?  If not, the books will push over the loco. ..

They have rubber feet to prevent them from sliding, and weigh about half a kilo each. A good weight when combined with rubber feet.

Interesting idea. I love the execution. Too bad my shelves are so full, I don't need bookends.

Just place them up against a wall instead :)

Those look great!  You could also work in a couple of Lego weighted bricks hidden in the base or tunnel sections to make them less likely to shift around.  How complete is the engine and passenger car?  Do they abruptly end where they enter the tunnel, or are each removable and complete in their own right?

The train and tunnel are built together as one piece to make the structure stronger.

These are really cool

I think these are great and have supported already on LEGO ideas. I appreciate the look as well as the stability of a model like this. As for the rubber feet, which pieces are those?

Thanks for the support! The rubber feet are part 14149.

Very cool idea and well executed, they look great 

Awesome!  These fit IDEAS criteria perfectly - a unique idea, that looks good alone, never before seen on IDEAS (in the review stage at least).  I’m pleased to say I supported them before seeing this post.  Hoping for a successful future on IDEAS for this project.  I would certainly buy!