I have a couple of issues needing to be brought up.
First: REMEMBER you have water in your siphon tube when storing you locomotive in cold weather. You may find you need a new siphon tube come spring time.
Second: The reason for a minimum of 20% and preferably 50% maximum gauge pressure over normal operating pressure is two fold: 1. Ensure the gauge will read over normal and not fail. 2. Just as important, bourdon tube gauges are most accurate near the middle of it's rated pressure range.
3. Siphon tubes are a good practice and have proven themselves over the years. Another option, although it has some drawbacks, is a snubber. A snubber performs the same functions as a siphon but with a higher safety factor. Refineries use gauges equipped with snubbers for high temperature and/or high pressure applications. The snubber completely isolates the gauge from the pressure source but still allows accurate transfer of pressure to the gauge. Drawbacks: 1. Not prototypical and will stand out like a sore thumb in a locomotive cab. 2. Expensive. Assets: 1. Greatest safety margin of any gauge protection. 2. Prevents the measured medium from entering the gauge.
Small Print: I am not advocating the use of snubbers in live steam locomotives. My goal is to contribute to the collective knowledge of this esteemed forum. I have enjoyed and benefited from others on this forum and it's time I paid some of it back.
Good Luck,
Terry