The file 'SpeedTrap – ‘Curved Version’ N Scale Speedometer…. 3D Printer Model' is (stl) file type, size is 258.5KB.
Curved version of the SpeedTrap speedometer that can be placed on a 'curved' (12.5" radius) section of N scale track and used to measure the loco's speed in MPH or KPH. There is also a straight version here....
https://www.thingiverse.com/thing:6701840
Much more about it with pictures, parts list and how to build it here....
https://1fatgmc.com/RailRoad/DCC/page-30.html
A video of how it works here....
https://www.youtube.com/watch?v=oAGohxnNBME
Another video of the 'straight' and 'curved' SpeedTraps next to each other...
A video showing the MPH and KPH options.....
https://www.youtube.com/watch?v=S4EAnjUjnb0
You can find other 3D printable parts for your layout here....
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50_Top_of_Case-1.stl | 65.8KB |