Guard Rails

In message , Puckdropper writes You can find all the information you need on the NMRA site at nmra.org Look at the various standards which will give you the required dimensions. If you are working in HO scale you can even buy a gauge to use for track laying.
Hope this helps

I used code 70 rails on code 83 flex track and placed them against the 'nubs' (moulded rail spikes) on the flextrack. Be sure to bend the ends towards the center. The point of these guard rails is to limit derailment and keep cars either on the rails or very close to staying on the rails and thus prevent cars from falling off the bridge and into the water (river, lake, swamp, etc.).

For guard rails on bridges and so forth, the purpose of the guard rail is to hold the wheelset in the area of the track and thus the wheel should be able to fall to the ties and contain it there. The guard rail on a turnout is intended to keep the opposite wheel from picking the point of the frog and thus it needs to contact the flange itself to control the position of the wheelset.
-- Bob May
rmay at nethere.com http: slash /nav.to slash bobmay http: slash /bobmay dot astronomy.net

Which NMRA standard - S-3.2 (Scale Track For Guarded Work)?

Nomenclature problem. Hope the following helps clear things up. I found OP's question obscure, as all he said was that he wasn't talking about "guard rails" in turnouts (these are usually termed "check rails.")
Guard rails: found on bridges, are designed to keep derailed trucks from wandering far enough sideways to cause serious damage to the bridge, tumbling the car off the bridge, etc. Standards vary somewhat, but generally they are spaced about 1 ft from the running rails. Study photos to get a sense of what will look right.
Guard rail: in track, are designed to help ease (if that's the word) rolling stock around curves. They are installed next to the inside rail of the curve, and prevent the wheels on the outside curve from climbing that rail. The flange way should be a little wider than that of a check rail, and will vary with the severity of the curve. Actually, typical model railroad curves are so sharp that the equivalent prototype curves would have guard rails installed.
Check rail: the rail opposite the frog in a turnout, designed to prevent the wheels from picking the frog and running down the wrong side of it. Spacing depends on wheel standards. NMRA
Grade crossing may have rails spiked down next to the running rails, but their purpose is to keep asphalt and gravel out of the flangeways.
HTH wolf k.

Wolf K wrote in news:4a9b43eb$0$24749$ snipped-for-privacy@news.newshosting.com:
I'm working with something like the "guard rail" definition above for curves. There's an expansion rail on one side (the rail's cut in half along its length and allowed to move back and forth as the layout expands), and the guard rail is across from it. The purpose is to keep the truck from falling off the non-expansion rail should the wheel fall in to the gap.
Puckdropper

Cut "along its length"?! The rail expands primarily along the length. Gapping the rail (a cut Across the rail) is the usual method of accommodating expansion and I've not seen guard rails used at the gaps.

In message , Bob May writes Here's a few photos taken in Canada of actual track with various check and guard rails. Perhaps the knowledgeable will identify each type for us :-)
First a bridge A 'normal' point A couple at a trestle bridge At a grade crossing A point (switch) with one check rail only. The frog is a 'self checking' one used where there is a maximum speed of 10 mph A fully self checking frog on a point

So which is the NMRA standard(s) which cover this, as Mike Hughes mentioned?

Check gauge, back-to-back, and flange way. NMRA gauge has go/no go tabs and notches to measure these. An essential tool IMO.
HTH wolf k.