Train with space for electronics and batteries in carriage.

This train engine has space inside for some electronics, a 10mm led can fit in the front, and a 16mm button on the smoke stack, there is room to slide an Arduino/Compatible Pro-Mini inside the cab and into the boiler area, and in my one I had just enough more room to squeeze a JQ6500-28 MP3 SD Card player module and a mini speaker in the cab. What you do with yours is your business, but mine when you hit the button it plays a random train sound effect or train-related kids song while changing random colours on the 10mm RGB led.

Such fun, I'm knocking on the door of 40 and I still can't resist pressing that button!

A piece of old USB cable of 5mm diameter is used as a power cable between the cab and the carriage which has enough room for 4 AA batteries and a mini boost converter if necessary (in mine I used 4 cells in 2S2P for (alkaline cells) 3v "full" down to 1.6v when "flat" and boost to 5v).

The "wheels" are fixed in place, I made this for an under-2-year-old so didn't want small/delicate wheels.

The roofs screw on, you'll probably need to drill the holes to fit your screws, up to 3mm diameter should work.

Notes: the coupling pin on my print broke off when removing support, so I drilled it out and used an M4 nut and bolt, however the USB cable actually makes a not bad linkage itself. I had some problems with wheel ovality at the bottom half due to support not being very well generated in slic3r for it.

Original locomotive design is from: http://www.thingiverse.com/thing:702858, edited in Tinkercad. I added the fixed wheels, hollowed it out, sliced off the roof, added screw fixing points, and removed the cow-catcher because it looked a bit pointy for little kids.

After printing I coated the underside with 5min Epoxy to give extra strength and make it slide easier. The front also got a coat of epoxy when the LED was installed to hold it in place.

Liberal hot glue inside to keep the electronics in place.

Painted with acrylic and varnish.

This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/2rXZmPWC1gv