My Dapol M7 derails on Peco N gauge setrack points

I struggled with this problem and judging by comments on other websites so have many others. The problem manifests when travelling bunker first through the curved side of the turnout. It either derails at the switch blade, or at the frog, or both.

When the bogie wheel meets the switch blade it rides up onto the top and falls off the other side (derail) and if it did not derail there then when it approaches the frog the wheel misses the guide rail taking the engine towards the other route through the frog (derail).

The cause does not lie with the turnout but with the M7 trailing bogie but, the solution requires modification to the turnout and M7.

There is some discussion elsewhere about the bogie swivel action being impeded by the electrical pick ups (this will not mean much to anyone who is unfamiliar with model) and/or insufficient weight over the bogie. But the truth is the bogie will go where the wheels take it.

The main causes are small wheel flange (does not always catch the guide rail), rocking motion of the axle (pivots on a spring that encourages the wheel to ride up any obstacle) and back to back wheel dimension.

Solutions

1. Reset the back to back dimension of the bogie wheel set to 0.275" (7mm) or thereabouts. This is narrower than the standard for N gauge but the wide tread of the M7 ensures the engine stays on the track and passes through the turnout.

2. Ensure the electrical pick ups for the wheels stick out equally each side of the bogie chassis so that the axles sit central when installed.

3. Reduce the rocking motion of the axle with a slither of plastic the width of the slot and place between the axle and pivot spring. It still needs to rock a little or the axle rotation will cease up.

4. Stick a thin strip of plasticard 1 mm tall to the top of the guide rail on the curved side of the turnout. On the underside of the M7 next to the  inside face of the large driving wheels shave a little off the plastic bits that rise from the chassis there. Run the engine through the turnout by hand and file down the raised guide rail until there is no interference. Leave at least 0.5 mm of the raised portion. Photo shows the modification.

On one of the turnouts I had to add a new guide rail positioned just before the switch blade on the curved rail side.

5. Check the M7 and other rolling stock traverse the turnout without derailing or sticking. Adjust as necessary.

The M7 will now be much more reliable but the occasional derail may still occur as the light weight of the engine comes into play.