You need a siding with either an invisable signal at the very end or a bufferstop. next get an unloading ramp (buildings tab) and place it at the end of the siding also. Finally in the bulidings tab select a Lars industry (un)loader , place it in the siding at a suitable distance from the unloading ramp and set it up to load or unload trailers. Trailers are in the vehicles section at the bottom of the commodities list. The idea is to have the rear of the train adjacent to the ramp to make it more realistic. Simples! (You probably have to live in the UK to understand the joke behind "Simples"
