Remember many of the trestle style coaling stations were built in the early days of railroading. Engines were small, labor was cheap. The first were indeed simple raised platforms on which coal was shoveled from a gon, then when an engine needed coaling the crew shoveled coal form the platform to the tender. As engines got larger the trestles got taller, and bins replaced the platforms. Many were covered. One or more gons or hoppers were shoved up the trestle and coal was shoveled or dumped into the bins. The bins had chutes on the side so an engine could be coaled by spotting the tender under the chute. The coaling station with a trestle was used by many railroads up into the 20s. But I believe they are all gone, replaced with coaling towers. As engine size increased, the appetite for coal grew, but also the distance an engine could go on one load of coal. Railroads began replacing their trestle coaling stations with coaling towers for faster more efficient coaling of locomotives. The first were built out of wood, but then concrete and steel. Some of these concrete behemoths still stand, no longer used and stripped of their metal parts.