Self-Guided LEGO Vehicles

A few weeks ago my local LEGO User Group (ParLUGment), set up a display at the Ottawa Train Expo. I was challenged by one of the other members to build a self-guided vehicle to drive itself through the layout, modeled after the Faller Car System. The Faller system is a common system used in traditional model layouts. It uses a magnet attached to the steering system of the vehicle to follow a wire embedded in the roads of the layout.

The concept is actually quite simple, and adapting it to a LEGO vehicle was pretty straight forward in principle. In the end there were a few issues that I encountered, which I will discuss below.  Be sure to watch the video to see it in action.

Being based in Ottawa, the obvious choice for vehicle was an OC Transpo bus.  I ended up building two buses, based on the ones currently in service: an Invero D40i and an Orion VII Hybrid.

OC Transpo Buses

Vehicle Frame

As I mentioned in the video, both buses were built on a common frame, which you can find instructions for here:


Building Guide

PDF File

Digital Model Files

LDraw File

Additional Resources

Parts List


There are a few important things to note about the frame.

1. I found it was critical to only provide power to one of the rear wheels.  When I was powering a single drive axle connected to both wheels the bus was predisposed to drive in a straight line (due to not having a differential). This caused a lot of problems with the steering, either continuing straight when going into a turn or over steering (jackknifing the steering system) while it was in a turn.

2. The frame can only turn sharp enough to go around LEGO road curves in one direction.  The magnet is attached to one side of the steering system and as a result the left and right turning radius are significantly different.

3. The magnet holder has to be oriented with the stud facing down, otherwise it will catch on the seams between road plates.  Of course if you are willing to damage your parts, you can just file down the top and bottom of the magnet holder to avoid any problems.

I built this frame with a design goal of using stock LEGO components, but if you are willing to modify parts, or use other materials, then you can otherwise overcome some of these issues.

Road Plates and Wire

The wire I’m using is just some picture hanging wire I had around the house.  It’s basic steel wire, about 1 mm thick.  Here’s a picture of the underside of a couple of the road plates.  It was pretty straight forward to tape the wire underneath.

Wired Baseplates

I found it was helpful to tape the seams where the road plates met with clear tape.  This helps prevent the magnet from getting stuck on the seams where they don’t quite meet at the same height.  You can see some of these pieces of tape in the video.

Also keep in mind that if you are running a vehicle for an extended period of time (like, many hours) the magnet sliding along the road plates will eventually start to wear through the paint.  I found that covering the magnet with clear tape minimized this issue.  If you are overly concerned about wear you could tape the entire line on the road plates where the magnet runs.  This way the magnet will only be running on the tape and not the road plate surface itself.

General Performance

In the controlled environment of my nice flat build table the performance of this system is 100%.  In the field this may vary, especially if the road loop spans multiple tables that aren’t quite level, or the tables aren’t perfectly flat (cheap plastic tables).  For our show at the Ottawa Train Expo it did require some attention (thanks Jeff!) when it occasionally ‘jumped the wire’.  At least this failure scenario isn’t catastrophic.  The vehicles don’t have enough power to drive over the studded parts of the baseplates, so they won’t go flying off the tables.


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