Motorized LEGO Technic Train Bogie with Buffers
This has the same size, wheel distance and buffer placement as the official train motor. But since the drive-axle comes up, you can use a larger motor instead, or add gear transmission for extreme speeds.
Because the build is Technic, it's very sturdy and won't fall apart. The buffers are also held on tightly, despite how it might initially look. I made this for an Arduino controlled train project, where I'm using a Power Functions L motor. This setup allows for better torque at lower speeds than the old train motor.
The building instructions contain not just the motorized bogie, but a regular one as well, along with a motor mount. The motorized and regular bogies are perfectly in level.
You can download the building instructions from here: