If you have a new model N15, you'll notice that the model is quite a beauty, the detail is superb, but the detail parts provided can interfere with the operation of the locomotive, and the front bogey is unsprung often resulting in de-railments.
First off this is my second N15, my first suffered an accident and was returned to Hattons, for a refund, or rather this N15. So I have some past experience

This is the best picture I could find of my modification. The draincocks were cut just after the first vertical bar in them, so that the wheels didnt get pushed off the rails when going around curves. This modification allowed operation on 2nd radius curves with the parts fitted.

I have no photos of springing the bogey, but I have my own directions:
1. Remove the bogey from the main chassis
2. Unscrew the central shaft from the lug nut
3. Place the central shaft, slotted bar, and lug nut to the side and find a suitably "springy" spring, preferably 1/3 of a bachmann one.
4. Take a little bit of superglue or any other glue, depending on how long you want it to sit for, and put it into the hole where the lug nut goes. Place the lug inside being careful NOT to get glue on the inside of the threads. This just makes it easier to screw in the central shaft again.
5.Place the spring between the bogey and slotted bar with the central shaft inserted into both
6. Screw the central shaft into the bogey, and test "springyness"
7. Re attach bogey to chassis, and test the loco, some springs are too stiff and elevate the first set of drivers, if this is the case, loosen the screw holding the bogey to the chassis, or take the whole thing apart and cut the spring.
This should solve most of the random derailments caused by the bogey not having any pressure on it.
For Etarre I had planned to fit the detail after i was set on my curve radius, but then decided to fit the loco first as it will be the main tester for clearances.
Stay tuned for my final modifications to Etarre, which allow it to run with the steps and draincocks fitted.
(only because I need to go eat

