(1) Gross Tonne-Kilometres (GTK): This term refers to the product of the total weight (in tonnes) of the trailing tonnage (both loaded and empty railcars) and the distance (in kilometres) the freight train travelled. It excludes the weight of locomotives pulling the trains. Units can also be expressed in gross ton-miles (GTM). (2) Revenue Tonne-Kilometres (RTK): This term refers to the product of the weight (in tonnes) of revenue commodities handled and the distance (in kilometres) transported. It excludes the tonne-kilometres involved in the movement of railway materials or any other non-revenue movement. The units can also be expressed in revenue ton-miles (RTM). (3) Passenger-Kilometres per Train-Kilometre: This term is a measure of intercity train efficiency, that is, the average of all revenue passenger kilometres travelled divided by the average of all train kilometres operated. (4) Revenue Passenger-Kilometres (RPK): The total of the number of revenue passengers multiplied by the distance (in kilometres) the passengers were transported. The units can also be expressed in revenue passenger-miles (RPM). (5) Emission Factor (EF): An emission factor is the average mass of a product of combustion emitted from a particular locomotive type for a specified amount of fuel consumed. The respective constituent emissions from a specific locomotive type are calculated based on data from test measurements, the operational duty cycle and engine specific fuel consumption. The EF units are grams, or kilograms, of a specific emission product per litre of diesel fuel consumed (g/L). (6) Emissions of Criteria Air Contaminant (CAC): CAC emissions are by-products of the combustion of diesel fuel and impact on human health and the environment. The principal CAC emissions are: (a) NO_x (Oxides of Nitrogen): these are the products of nitrogen and oxygen that result from high combustion temperatures. The amount of NO_x emitted is a function of peak combustion temperature. NO_x reacts with hydrocarbons to form ground-level ozone in the presence of sunlight to contribute to smog formation. (b) CO (Carbon Monoxide): this toxic gas is a by-product of the incomplete combustion of fossil fuels. Relative to other prime movers, it is low in diesel engines. (c) HC (Hydrocarbons): these are the result of incomplete combustion of diesel fuel and lubricating oil. (d) PM (Particulate Matter): this is residue of combustion consisting of soot, hydrocarbon particles from partially burned fuel and lubricating oil and agglomerates of metallic ash and sulphates. It is known as primary PM. Increasing the combustion temperatures and duration can lower PM. It should be noted that NO_x and PM emissions are interdependent; that is, technologies that control NO_x (such as retarding injection timing) result in higher PM emissions. Conversely, technologies that control PM often result in increased NO_x emissions. (e) SO_x (Oxides of Sulphur): these emissions are the result of burning fuels containing sulphur compounds. For the LEM reporting, sulphur emissions are calculated as SO_2. These emissions can be reduced by using lower sulphur content diesel fuel. Reducing fuel sulphur content will also typically reduce emissions of sulphate-based PM. (7) Emissions of Greenhouse Gases (GHG): In addition to CACs, GHG emissions are also under scrutiny due to their accumulation in the atmosphere and contribution to global warming. The GHG constituents produced by the combustion of diesel fuel are: (a) CO₂ (Carbon Dioxide): this gas is by far the largest by-product of combustion emitted from engines and is the principal ‘greenhouse gas’ which, due to its accumulation in the atmosphere, is considered to be the main contributor to global warming. It has a Global Warming Potential of 1.0. CO₂ and water vapour are normal by-products of the combustion of fossil fuels. The only way to reduce CO₂ emissions is to reduce the consumption of fossil fuels. (b) CH₄ (Methane): this is a colourless, odourless and inflammable gas that is a bi-product of incomplete diesel combustion. Relative to CO₂, it has a Global Warming Potential of 21. (c) N₂O (Nitrous Oxide): this is a colourless gas produced during combustion that has a Global Warming Potential of 310 (relative to CO₂). The sum of the constituent greenhouse gases expressed in terms of their equivalents to the Global Warming Potential of CO₂ is depicted as CO₂ equivalent. This is calculated by multiplying the volume of fuel consumed by the Emission Factor of each constituent then, in turn, multiplying the product by the respective Global Warming Potential, and then summing them. See page xi for conversion values pertaining to diesel fuel combustion