How do I test the width between the wheels of my engines?

The best way to check the wheels of your locomotives is to use a tool called a back to back gauge.

They are available for all scales/gauges. They are made of brass and you simply slide it over the axle and the back of the wheels.

If it proves difficult to get your gauge over the axle and between the wheels, the gap is too tight, then the wheels are too close together, so the axle will need removing from the chassis to enable you to pull the wheels apart slightly to allow the back to back gauge.

If the gauge does not touch both wheels, the wheels are too far apart and need to be squeezed back together.

Hopefully one of these in your tool kit will give you many years of perfect rolling and stop those annoying derailments.