There is now a newer and better version of this thing.

This is inspired by MilanGajic's electric locomotive. I wanted it to be as compact as possible (to fit in the Lillabo tunnel) while looking like a (kid's idea of a) locomotive (and in particular having a nice cow plow at the front end!). I still consider this as work-in-progress, see below.

Good points of this model:

• it works!
• it (just) fits under the Lillabo tunnel!
• parts just snap together, but could also be held by 3mm bolts (either directly into the plastic, or in heat-set inserts — the space for these is also designed into the model).

Points that still need to be improved:

• wheel adhesion: this can only pull two to three cars and cannot even pull itself over the Lillabo bridge. (Adding an elastic should probably be enough for this, but this needs slightly redesigning the wheels).
• the electric circuit (which I copied from the original) is not optimal: the LED is in series with the motor. So whenever the wheels are blocked (for example because you just picked it up), the DC motor becomes a passing circuit, which shorts (and kills) the LED. To fix this, the LED should be in parallel with the motor, and in series with a 100/200 ohm resistance (for blue/yellow LEDs), which might need some extra space in the model.
• the switch is a toggle hidden in the chimney, but this could be made even more elegant (and easier for kid's fingers) by making the whole chimney the switch — either rotary or as a (latching) push-button switch. I have not yet located the correct part for this on AliExpress, so in case anyone identifies a good latching push-button (with ~ 7mm hole) or rotary switch, feel free to send me the link!

Extra parts needed:

• small toggle switch

• T-shape DC motor

• small lithium battery and micro-USB charging module

• 12mm ring magnet and round-head 4mm bolt & nut

• 2mm wide axle for the front wheel (needs to be sawed to 33mm width)

(I used a steel axle because some Brio accessories (level crossing) use magnets to detect the axle. The model could also use a printed plastic axle, attached to the front wheels, at the cost of a very slight redesign of the front wheel attachment to the basis).

• optionally, 3mm bolts and heat-set inserts.