Took the synopses provided by Wikipedia in a convenient, tidy list, good for printing as a quick reference guide.

Ballast forms the trackbed upon which ties are laid. It is packed between, below, and around the ties. It is used to bear the load from the railroad ties, to facilitate drainage of water, and to keep down vegetation that might interfere with the track structure. This also serves to hold the track in place as the trains roll by.
A bogie is a chassis or framework carrying wheels, attached to a vehicle, thus serving as a modular subassembly of wheels and axles.
railcar, wagon
A car is a vehicle used for the carrying of cargo or passengers on a rail transport system. Such cars, when coupled together and hauled by one or more locomotives, form a train. Alternatively, some passenger cars are self-propelled, in which case they may be either single railcars or make up multiple unit.
A coupling is a mechanism for connecting rolling stock in a train.
Diesel Locomotive
A diesel locomotive is a type of railway locomotive in which the prime mover is a diesel engine.
Driving Wheel
On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons. On diesel and electric locomotives, the driving wheels may be directly driven by the traction motors.
Dynamic Braking
Dynamic braking is the use of the electric traction motors of a railroad vehicle as generators when slowing the locomotive.
Electric Locomotive
An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or on-board energy storage such as a battery or fuel cell.
Electrification System
A railway electrification system supplies electric power to railway trains and trams without an on-board prime mover or local fuel supply.
Gauge is the spacing of the rails on a railway track and is measured between the inner faces of the load-bearing rails.
The grade of a physical feature, landform or constructed line refers to the tangent of the angle of that surface to the horizontal.
Heritage Railway
A heritage railway is a railway kept to carry living history rail traffic to re-create or preserve railway scenes of the past. Often heritage railways are old railway lines preserved in a state which depicts a certain period, or periods, in the history of railway systems.
The livery is the common design and paint scheme a company will use on its vehicles, often using specific colors and logo placement.
engine, loco
A locomotive is a rail transport vehicle that provides the motive power for a train.
Multiple Unit
Multiple units are self-propelled train carriages capable of coupling with other units of the same or similar type and still being controlled from one driving cab. Often these are passenger trainsets consisting of more than one carriage.
Overhead Line
catenary, overhead contact system, overhead equipment, overhead line equipment, overhead wire, overhead wiring, traction wire, trolley wire
An overhead line is used to send electrical energy to trains. The overhead line is designed on the principle of one or more overhead wires situated over rail tracks, raised to a high electrical potential by connection to feeder stations at regular intervals.
Passenger Car
carriage, coach
A passenger car is a piece of railway rolling stock that is designed to carry passengers.
Rail Transport
Rail transport is a means of conveyance of passengers and goods on wheeled vehicles running on rails, also known as tracks.
A railyard is a complex series of railway track for storing, sorting, loading, and unloading railroad cars and locomotives. Railyards have many tracks in parallel for keeping rolling stock stored off the mainline, so that they do not obstruct the flow of traffic. Railroad cars are moved around by specially designed yard shunters, a type of locomotive.
Rolling Stock
Rolling stock refers to any vehicles that move on a railway, including locomotives, coaches, and wagons.
A sandbox is a container on most locomotives, multiple units and trams that holds sand, which is dropped on the rail in front of the driving wheels in wet and slippery conditions and on steep grades to improve traction.
shifter, switch engine, switcher, yard goat, yard pilot
A shunter is a small railway locomotive intended not for moving trains over long distances, but rather for assembling trains ready for a road locomotive to take over, disassembling a train that has been brought in, and generally moving railroad cars around – a process usually known as shunting.
Shunting, in railway operations, is the process of sorting items of rolling stock into complete trains, or the reverse.
A signal passed at danger occurs when a train passes a stop signal without authority to do so.
Steam Locomotive
A steam locomotive is a railway locomotive that produces its pulling power through a steam engine. These locomotives are fueled by burning combustible material, usually coal, wood, or oil, to produce steam in a boiler. The steam moves reciprocating pistons which are mechanically connected to the locomotive's driving wheels. Both fuel and water supplies are carried with the locomotive, either on the locomotive itself or in tenders pulled behind.
The subgrade is the native material under a railroad track.
A tender is a special rail vehicle hauled by a steam locomotive containing its fuel and water.
Third Rail
A third rail is a method of providing electric power to a railway locomotive, through a semi-continuous rigid conductor placed alongside or between the rails of a railway track.
crosstie, sleeper
A tie is a rectangular support for the rails in railroad tracks. Generally laid perpendicular to the rails, ties transfer loads to the ballast and subgrade, hold the rails upright and keep them spaced to the correct gauge.
The track on a railway or railroad is the structure consisting of the rails, fasteners, railroad ties, and ballast, plus the underlying subgrade. It enables trains to move by providing a dependable surface for their wheels to roll upon. Tracks where electric locomotives or electric trams run are equipped with an electrification system such as an overhead electrical power line or an additional electrified rail.
consist, trainset
A train is a form of rail transport consisting of a series of vehicles that usually runs along a track to transport cargo or passengers. Motive power is provided by a separate locomotive or individual motors in self-propelled multiple units.
streetcar, tramcar, trolley
A tram is a rail vehicle which runs on tracks along public urban streets.
A wheelset is the wheel-axle assembly of a railroad car. The frame assembly beneath each end of a car or locomotive that holds the wheelsets is called the bogie.