Sometimes it really pays to watch what non-train LEGO fans are up to.
This tool is super simple to use. Find an image online, enter the URL, adjust the image, then draw a known dimension. After the Calculate button is clicked, all other dimensions drawn on screen follow the scale set with the original line.
As an example, in the image above, I defined the first line, drawn across the middle driver wheel of the 2-6-0 locomotive, as 4 studs. A few quick lines later, and I can easily see that the complete locomotive is 27 studs long, the tender is 14 studs long, and the cab is 10 studs tall from track level.
I've only played with this for a few minutes, but I can already see it becoming an standard tool for LEGO modelers of all sorts.