At present it's possible to hold down the mouse over certain gui-toolbar buttons to get a drop-down menu of options (i.e., holding down over the rail button gives a choice of rail types to build with). But graphically, there's no way to know this functionality exists or even which buttons have it. They all look identical and it's just trial and error, which I believe is generally a no-no for UI design.
So my suggestion - do something like this (or whatever - something to differentiate them): Note the little arrow next to the railway icon (nabbed straight from Open Office
