Try learning a foreign language, the Queen's English! Coaching stock means passenger cars over here. SuperSpeedMaglev is looking for models of the British Royal Train and I personally do not know of any in Trainz.
Coaching stock is not only passenger carriages, there is also a huge variety of Non-Passenger Carrying Coaching Stock (NPCCS). These include, amongst others, such things as: pigeon vans, travelling post offices, bullion vans, milk tankers, horse boxes and closed carriage trucks.