Given the question: Information:  - The Snaefell Mountain Railway is an electric mountain railway on the Isle of Man in Europe. It joins the village of Laxey with the summit of Snaefell, at above sea level the highest point on the island. It connects with the Manx Electric Railway (MER) in Laxey. The line is long, built to gauge and uses a Fell Incline Railway System centre rail for braking on the steep gradients. It is electrified using overhead wires at 550 volts direct current, with bow collectors.  - The volt (symbol: V) is the derived unit for electric potential, electric potential difference (voltage), and electromotive force. The volt is named in honour of the Italian physicist Alessandro Volta (17451827), who invented the voltaic pile, possibly the first chemical battery.  - Mean sea level (MSL) (abbreviated simply sea level) is an average level of the surface of one or more of Earth's oceans from which heights such as elevations may be measured. MSL is a type of vertical datuma standardised geodetic reference pointthat is used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured in order to calibrate altitude and, consequently, aircraft flight levels. A common and relatively straightforward mean sea-level standard is the midpoint between a mean low and mean high tide at a particular location.  - The Isle of Man, also known simply as Mann, is a self-governing crown dependency in the Irish Sea between England and Northern Ireland. The head of state is Queen Elizabeth II, who holds the title of Lord of Mann. The Lord of Mann is represented by a Lieutenant Governor. Foreign relations and defence are the responsibility of the British Government.  - Laxey is a village on the east coast of the Isle of Man. Its name derives from the Old Norse "Laxa" meaning 'Salmon River'. Its key distinguishing features are its 3 working vintage railways, having the largest working waterwheel in the world, and being set in a partly wooded valley.  - A rack railway (also rack-and-pinion railway, cog railway) is a steep grade railway with a toothed rack rail, usually between the running rails. The trains are fitted with one or more cog wheels or pinions that mesh with this rack rail. This allows the trains to operate on steep grades above around 7 to 10%, which is the maximum for friction-based rail. Most rack railways are mountain railways, although a few are transit railways or tramways built to overcome a steep gradient in an urban environment.  - A pinion is a round gear used in several applications:  - Snaefell is the highest mountain and the only summit higher than on the Isle of Man, at above sea level. The summit is crowned by a railway station, cafe and several communications masts.  - A tram (also known as tramcar; and in North America known as streetcar, trolley or trolley car) is a rail vehicle which runs on tracks along public urban streets, and also sometimes on a segregated right of way. The lines or networks operated by tramcars are called tramways. Tramways powered by electricity, the most common type historically, were once called electric street railways (mainly in the USA). However, trams were widely used in urban areas before the universal adoption of electrification; other methods of powering trams are listed below under "History".  - A mountain railway is a railway that operates in a mountain region. It may operate through the mountains, for example by following mountain valleys and tunneling beneath mountain passes. Or it may climb mountains, in order to provide transport to and from their summits.  - The Manx Electric Railway is an electric interurban tramway connecting Douglas, Laxey and Ramsey in the Isle of Man. It connects with the Douglas Bay Horse Tramway at its southern terminus at Derby Castle at the northern end of the promenade in Douglas, and with the Snaefell Mountain Railway at Laxey. The line is undulating and passes through areas of scenic beauty. Many visitors take an excursion on the trams.  - A train is a form of rail transport consisting of a series of vehicles that usually runs along a rail track to transport cargo or passengers. Motive power is provided by a separate locomotive or individual motors in self-propelled multiple units. Although historically steam propulsion dominated, the most common modern forms are diesel and electric locomotives, the latter supplied by overhead wires or additional rails. Other energy sources include horses, engine or water-driven rope or wire winch, gravity, pneumatics, batteries, and gas turbines. Train tracks usually consist of two running rails, sometimes supplemented by additional rails such as electric conducting rails and rack rails, with a limited number of monorails and maglev guideways in the mix. The word 'train' comes from the Old French "trahiner", from the Latin "trahere" 'pull, draw'.  - A bow collector is one of the three main devices used on tramcars to transfer electric current from the wires above to the tram below. While once very common in continental Europe, it was replaced by the pantograph or the trolley pole, itself often later replaced by the pantograph.  - The Fell system was the first third - rail system for railways that were too steep to be worked by adhesion on the two running rails alone . It uses a raised centre rail between the two running rails to provide extra traction and braking , or braking alone . Trains are propelled by wheels or braked by shoes pressed horizontally onto the centre rail , as well as by the normal running wheels . Extra brake shoes are fitted to specially designed or adapted Fell locomotives and brake vans , and for traction the locomotive has an auxiliary engine powering horizontal wheels which clamp onto the third rail . The Fell system was developed in the 1860s and was soon superseded by various types of rack railway for new lines , but some Fell systems remained in use into the 1960s . The Snaefell Mountain Railway still uses the Fell system for ( emergency ) braking , but not for traction .  - Direct current (DC) is the unidirectional flow of electric charge. Direct current is produced by sources such as batteries, power supplies, thermocouples, solar cells, or dynamos. Direct current may flow in a conductor such as a wire, but can also flow through semiconductors, insulators, or even through a vacuum as in electron or ion beams. The electric current flows in a constant direction, distinguishing it from alternating current (AC). A term formerly used for this type of current was galvanic current.    After reading the paragraphs above, we are interested in knowing the entity with which 'fell mountain railway system' exhibits the relationship of 'subclass of'. Find the answer from the choices below.  Choices: - aircraft  - average  - aviation  - battery  - bay  - bow  - cartography  - conductor  - electric charge  - electric current  - engine  - england  - flight  - french  - gauge  - governor  - gravity  - head of state  - ion  - irish  - key  - latin  - lieutenant governor  - line  - location  - man  - mountain  - name  - physicist  - point  - propulsion  - public  - rail  - railway station  - railway system  - responsibility  - river  - rope  - round  - running  - sea  - series  - state  - steep grade railway  - street  - surface  - symbol  - system  - term  - title  - track  - train  - tram  - valley  - vehicle  - village  - world
The answer is:
steep grade railway