W.
Wagon Tippler : A mechanical device which is able to lift up a whole wagon and tip out its contents.
Wagon Turntable : A short-length turntable consisting of a round plate on which two tracks at right angles to each other are located. These devices are used for maneuvering individual wagons between tracks which are at too obtuse an angle to each other to be connected with points.
Waist (Panel) : The area of a carriage body about half-way up the body-side; specifically the narrow panels below the windows on a traditionally paneled coach.
Water Column : A standpipe adjacent to the track and connected to a water supply for filling steam locomotive tenders.
Water Crane : A water column on top of which an additional hinged horizontal pipe is attached.
Water Scoop : To save having to stop to take on more water, many steam locomotives were fitted with water scoops which could pick up water with out stopping from long troughs laid along the track between the rails.
Water Tower : An elevated water-storage tank.
Water Troughs : Longitudinal troughs of water laid between the rails of a train track, for the purpose of a locomotive picking up water without stopping.  
Weathering : Making shiny new models look more realistic by dirtying them up to simulate road grime, the action of sun and rain, and general evidence of use.
Weathering Powder : A means of selectively 'dirtying' the model to make it seem as if the model has seen years of active service!
Welded Rail : Real rail sections that have been welded together to eliminate track joints. Also called continuous rail.
Well Tank : Some steam locomotive carry their water supply in a tank set between the locomotive frames. As this greatly restricts the tank size, many locomotives with well tanks also had small side tanks.
Well Wagon : A goods wagon in which the portion between the two axles or bogies is lowered so as to provide room within the loading- gauge for a high load.
Wheel Base : The distance between the centres of the first and last axles of a vehicle.  
White Glue (PVA) : An adhesive primarily for joining wood, paper and card materials.
Wireless Cab : A handheld cab that has no cable connection to the layout. Wireless cabs use infrared or radio waves as a method of transmitting information.
Working Timetable : A time table used by railway operators, which includes the times of all regularly-run trains (not just passenger trains).  
Wye : Type of turnout where the two legs curve away equilaterally, forming a Y. Also a triangular track configuration for turning a locomotive or a train or for joining a branch to a main line for operation in both directions.

X.

Y.

Z.