I could not help myself when I made the previous version I was thinking of doing this, but took the easy way out, and because of the interest in the MKI http://www.thingiverse.com/thing:2025266, here is the MKII.
The number of times I have had to trace along a piece of track to mark out where I want to lay the underlay, or marking out the curved form-work to lay the track.
Of course one does not have a set of radius curves of every size so this simple device joined up with the OO/HO FLEXITRACK CURVE FORMING CLAMPS in http://www.thingiverse.com/thing:2023001 you can make a radius template any size you like, and is not limited to model railways uses either.
The pencil needs to slide easily in the holder and may need a bit of extra weight added.
Wheels are on a 2mm axle, but may need changing to suit what you have lying around
The axle slots are set at 2mm but will print tight so I used a 2.5mm drill run through the bottom of the slot so the axle needs to be pushed past the narrow bit and will snap into place.
The bogie is 13mm wide so may need to be adjusted to suit your axles.