I have a stamped steel sign that is 18" wide and 24" high.
It says: “1 MILE TO YARD LIMIT”
From “The Consolidated Code of Operating Rules”, 1959:
MAIN TRACK–A track extending through yards and between stations, upon which trains are operated by time-table or train order, or both, or the use of which is governed by block signals.
YARD–A system of tracks withing defined limits provided for the making up of trains, storing of cars and other purposes, over which movements not authorized by time-table, or by train order, may be made, subject to prescribed signals and rules, or special instructions.
A significant difference is that out on the main, an engineer knows he has the railroad IF he and others are following the rules mentioned above. He can go 50 without worrying that there’s something on the tracks ahead of him (well, maybe just a little–there sure isn’t SUPPOSED to be!). That sign I mentioned is telling him that he’s coming up to a place where he just might find equipment right in front of him, with no notice at all. And he needs to be going slowly enough that he can stop when he sees any.
Now that we’ve established the fact that your railroad is set in the 1930s, one thing you will need, anywhere it is likely for men to be on top of your house cars, wherever there is an overhead clearance issue, are telltales. This may be at a bridge or, a tunnel. The telltales would be placed over all tracks with low clearance, on both sides of the low clearance. They will also be placed far enough in advance of the overhead structure, to give adequate warning. As better brakes were developed, the reason for men going atop equipment was mainly to pass signals in switching, before radios came into use. The elimination of roofwalks on house cars also led to the demise of telltales.
Your no clearance sign(s) would appear where there is insufficient clearance for a man to ride on the side of a car or locomotive. These are usually found in industrial areas but could also be found on railroad installations as well. Common side clearance problems occur when tracks go through gates or, doorways, into buildings, or where tracks come close to the walls or corners of structures. Another close clearance situation could exist where tracks parallel loading docks. I have seen close clearance signs mounted on building walls at doorways and also on posts a couple of car lengths from the obstruction. Wording could simply be “close clearance” or something like “will not clear man on side of car”. If cars can move beyond the obstruction, you could have signs on both sides of it.
On a small layout - remembering even a large model railroad is a very compressed version of reality - it might be better not to put too many signs in place. Trying to squeeze in every possible sign could just make the layout look unrealistic and too ‘busy’. Think of it kinda like part of the scenery of the layout, add signs where appropriate and they look ‘right’.
Basic Scenery for Model Railroaders by Lou Sassi is a great book in general, but he has a chapter in there about making right-of-way signs from styrene. I believe he also has some information about placing them prototypically, but this might vary from railroad to railroad.
I can’t remember where I found them but I printed out scale traffic signs. I glued the sheet to some thin card stock to give them some stiffness. I then used an Xacto knife to cut out the sign. For my HO scale, I found that square tooth picks were very close to scale. Using a Xuron nipper, I cut the points off and glued the signs to the "posts. I then just used Elmer’s white glue to set them on the layout.