It's older locomotives that suffer from this mainly of the wren / Triang-Hornby era up to 1975. I have had occaisional problems with ringfield motors which have been fixed with a remag, but these tend to keep their magnetic field quite well. I've not come across a modern motor which suffers from this but it could do. Symptoms of weak motors are poor pulling performance and the armatures tend to get hot and can burn out. If you run a motor and it smells like it's getting hot, there may be a problem. As I said above, you can remove the body and use an app to measure the magnetic field. Alternately you can place a small screwdriver on the magnet and you should feel reasonable resistance pulling it off. If there feels to be little resistance, the magnet is probably weak. Some people buy 'super neo' magnets to replacethe originals and they do give a really strong field. I have read posts which say this puts a strain on the motor and damages it in the long term. I bought a remagnetiser off ebay and so remag my motors myself and this does work. There are lots of reasons why models stop randomly and almost certainly not the magnet. Could be dirt on the pickups, dirt on the track, odd pickups not contacting the wheels or losing contact on bends. If it's happening in the same place on the layout it's a track / pickup thing. If it's random, a service should help (lubrication and pickup cleaning).