Re: Boiler Pressure

Kevin:
Size of the boiler is really not much of a factor in choosing boiler pressure. Like most things, it is a trade off between taking advantage of the higher thermal efficiency and necessary boiler weight of higher pressure balanced against the simpler and less expensive to manufacture and maintain lower pressure. Even lubrication enters into the choice in that the steam temperature at 300 degrees pressure is so high that finding valve and cylinder lubricants that don't burn and carburize was a problem. The UP 800s were a testimonial to this.
By using a higher boiler pressure, it is possible to use smaller and lighter weight pistons and valve motion to achieve the same power output. This is especially important in balancing an engine running at high speed where as the narrow gauge engines were kept more economically simple due to lower operating speeds.
As you can see, these are only a few of a number of reasons to choose one over the other including the personal preference of the designers.