These are the M track curved points/turnouts.

Why is it that the small 'guard rail' becomes loose over time? I don't know its correct name but the bit near the white arrow in my picture. When it becomes loose derailment can result.

Are my old locos too heavy? Are they running too fast? Or is it simply a case of age?

It is easy to rectify but I seem to have to do it quite often. However it means removing the piece from the layout which inevitable causes problems elsewhere.

Any advice much appreciated.

point by dralowid, on Flickr

Hi Jean,

Many thanks. I am familiar with correcting the issue, as you say, it is easy once removed from the layout.

My real question is about the cause. Why does it happen?

Vibration.
I wonder if they can be soldered permanently from underneath? One can only tighten the little twisty bits so many times before the metal tears. I guess they never figured we would be playing with this stuff 70- 100 years later.

In the UK they are called 'Check Rails', I don't know if that name is used in other countries.

I've had a couple come loose and I just fixed them by dribbling a drop of superglue down the edge of the check rail at the position where the tongue goes down through the base of the point. I used a good axle to ensure the check rail was in the right position before it dried. Those fixes have held firmly ever since.
Using superglue means that it can be done from above so you don't have to disturb the track. Just don't run any trains until the glue has dried.
A couple of little dabs of epoxy on the edge where the rail contacts the base would probably work just as well.

It must have been a problem that was recognised by Marklin as later points have these check rails moulded in plastic in one with the sleepers which are a bit thicker where the check rail is.

It's a nuisance but easily fixable.

14 +/- 0.1 mm