One day I was staring at an empty wall, and I thought, wouldn't it be nice if I had a train that went on the wall.
It was not a rational thing to ask for, but I had science and technology in my bag, so I came up with a plan to defy gravity and friction.
Magnets provide the force, wheels covered with a special material provide the friction, and motors are geared and configured to be able to pull the whole thing against gravity.
To make a train go upside down is quite easy as opposed to going on the wall, vertically or horizontally. At first, I thought a pair of magnets would be sufficient. However this was not true. It needed ultra-high traction wheels, so I made the wheel with a special shape, and coated it with an elastomer material.
The whole design was done in sketch-up, and printed using a 3D printer. This was my personal hobby scale Apollo 11 project, where I had to show me, myself and I, that trains could go on the wall, if I wanted.