H.
Halt : A railway station with minimal facilities where trains will usually stop only on request.
Hand Points : A set of points which are actuated by an individual, trackside hand- lever.
Head Boards : Discs which when fitted to the lamp irons on the front of a train or locomotive indicate the head-code.
Head-Code : The arrangement of head-boards or head-lamps on the front of a train which indicate to a signalman, or other railway employee, the type of train, or route to be taken by the train.
Headshunt : A length of track which feeds any number of sidings and that allows those sidings to be shunted without blocking the main line.
Home signal : The semaphore signal controlling entry into a block section.
Hot Knife : Used for cutting polystyrene tiles. Just a piece of wire held in a special tool handle. It plugs into the electric mains, gets hot and melts the polystyrene tile as you move it through the material.
Hump Shunting : A system of shunting where vehicles are propelled up one side of a hump at the throat of a fan of sidings, and then allowed to run down the other side of the hump to their required location.
Hump Yard : A marshalling yard used in hump shunting.  
Hymek : A diesel hydraulic locomotive produced for the Western region of British Rail that were designated Class 35.

I.
Inner Home Signal : Where it is necessary to have two consecutive stop signals within station limits before the station platforms are reached in the normal direction of travel, the second of the stop signals is called the inner home. Where there are more than two home signals, they are usually numbered.
Insulfrog : As the name suggests these turnouts have an insulated tip to the frog, in other words, electrically dead. This keeps the two electrical circuits through the turnout completely separated and only requires insulated rail joiners to be fitted under certain circumstances.
Insulated Rail Joiner : To be used when rails need to be connected to each other mechanically but isolated from each other electrically.
Interlocking : A system of mechanical and/or electrical controls allowing only one train to move through a junction of two or more tracks.
Intermediate Block Section : A fully track-circuited block-section, located between two normal block sections. Each line of the intermediate block section is controlled by the signalbox of the block post in rear.
Island Platform : A platform which has through tracks on both sides.  

J.

K.
Kit built : A model built entirely from the parts supplied. An obvious one really but relevant to what follows.
Kit bashed : This is when you take a kit of some model and so alter or add to it that the model takes on your own personal stamp or may even become a totally different model.