Currently, there are no global standards defining the terminology and classification of types of urban railway systems according to certain criteria. Attribution to a particular type is mainly carried out on the basis of official information from the company-operator of a particular system or accepted criteria in a particular country. The lack of precise criteria and deviations leads to the fact that the system, for example, is called the term "Metro", but in fact it is not. This section is the author's version of the classification system that determines the criteria for the types of urban Railways and their key parameters (length of lines, number of stations). General-purpose Railways and typical (non-high-speed) tram systems are not considered in this review.

1. Generalities

Currently in the World, there are the following names of types of Urban Railway Systems:

    The names given here in different countries of the world can have different variations and transformations reflecting the local approach to the name of the systems. For example, Light Rail Transit may have the shorter name "Light Rail". The name Metro Rail Transit (MRT) may sound like a Metro Rail Transit (MTR) or Metro Rail Transit System (MRTS). Other specific systems such as Monorail, Suspension rail, Funicular, People Mover, Maglev and Translohr are also urban rail transport, but are not considered in the context of these detailed classification, as they have pronounced distinctive design features.

2. Description of the Systems by Types

Metro is an abbreviated word from the British Metropolitan. This name is the most often used in the world. Example: Moscow.

Underground - the term is used only in the United Kingdom to refer to urban underground Railways. In fact, it corresponds to the generally accepted term "Metro". Example: London.

Subway - the term is used in the United States and Scotland (England) to refer to urban underground Railways. In fact, it corresponds to the generally accepted term "Metro". Example: New York, Glasgow.

Mini-metro the system in fact has the same design as the Metro system, but uses a rolling stock of smaller passenger capacity and short trains. Example: Copenhagen.

Overground - the term is used only in the UK to refer to urban elevated Railways. This mode of transport is closely connected with conventional Railways, as it is built based on their modernization. Example: London.

U-Bahn - The term entirely corresponding to the term metro, but in Germany and Austria, it is additionally used to refer to all underground stopping points of urban Railways including commuter trains, high-speed trams and Light rail. Example: Berlin and Frankfurt am Main.

T-Bana - the term entirely corresponding to the term "metro", but is used in Scandinavian countries. Example: Oslo.

S-Bahn - the term fully corresponds to the term "Overground", but in Germany and Austria refers to the lines of suburban trains passing through the Central part of the city and operating in a mode similar to the subway. Example: Berlin.

Regional Express Rail (RER) this term very much on the characteristics of the "S-Bahn" and "Overground". In fact, it refers to suburban train lines, often running on conventional Railways, but having separate underground and surface sections of lines for accelerated passenger traffic in the city center. Example: Paris.

Suburban rail - the term means a system of lines of ordinary commuter trains using the tracks of public Railways that do not have Express traffic and have frequent intersections with the road network of the city.

Regional metro is essentially a symbiosis of the classic metro and suburban trains. As a rule, the rolling stock is very similar to subway trains, the lines run on the surface and are separated from the Railways, but go deep into suburban areas and have long distances between stations. The average distance between stations on the line is more than 3.2 km.*(see note). Example: San Francisco.

Metro Rail Transit (or Mass Transit Railway or Mass Rapid Transit Systems) - in most countries of the world (mainly in Asia), this term refers to the classic term "metro", but, for example, in Australia, USA, Canada, Africa and India in a number of cities, this term applies to commuter trains such as "S-Bahn", "RER" and "Overground". Example: Singapore.

Airport express - so called special lines or routes of urban Railways connecting the city center with the airport. In some cities, lines to the airport are characterized as part of the metro system, but in terms of rolling stock and charging, these lines differ markedly. Example: Delhi.

Light Rail Transit (or Light Rapid Transit) -  the term describes a system that is featured between the subway and tram. Separate from other rail lines, often crossing from terrestrial traffic and is used as a low so high platform. Light rail lines often follow the same tracks as tramlines. Example: Seville.

Pre-metro - the term fully converges with the term "Light rail". Example: Buenos Aires.

Speed tram - a term denoting tramlines or their individual high-speed sections that are separate from the road, but use rolling stock of conventional tram. Example: Volgograd.

Other systems:

The Monorail system uses a single guide rail and magnetic levitation trains (Link to Wikipedia page about Monorail systems).

The Maglev system uses one wide guide rail and high-speed magnetic levitation trains (Link to Wikipedia page about Monorail systems).

The Suspended rail - these lines use guide suspension monorail and track system over rolling stock (Link to Wikipedia page about these systems).

The Funicular - this transport is designed to move passengers to the hills on inclined tracks. Trains consist, as a rule, of no more than 2 short cars and have a special security system (Link to Wikipedia page about Funicular systems).

The People Mover - lines of this type of transport are usually very short and are built to serve certain facilities (airports, exhibitions, recreation areas). Rolling stock is fully automated, has a severely limited carrying capacity and is not a mass urban transport (Link to Wikipedia page about People Mover systems).

The Translohr - these rail systems are a symbiosis of bus and tram. The rolling stock is similar in appearance to a tram with an upper power supply, has pneumatic wheels for driving on simple roads. The direction of movement is set using a single guide rail mounted in the road surface (Link to Wikipedia page about Translohr systems).

* Note: the criterion is calculated by comparing two "boundary" (in terms of the average distance between stations) to other significant criteria of the systems San Francisco and Wenzhou. The BART system in the San Francisco region has only one small area in the city and then goes far into the suburbs. The lines use trains consisting of 10 cars with comfortable single seats and two exits on one side. Thus, the system is more like a suburban regional railway. The Wenzhou metro system has a slightly smaller average distance between stations. Similar to the BART system in San Francisco, the line goes far into the suburbs, but the rolling stock is similar to the classic subway and the cars have 4 exits on each side. Therefore, the conclusion is made about the attribution of Wenzhou metro and similar systems to the term "Metro".

3. Key parameters for classification of the Systems by the Type

     For each system, there are pronounced criteria, a set of which determines the attribution to a particular type of system. For simplicity, you can divide the types of systems into four main groups:

3.1  METRO

  The following parameters are defined for the Metro group, applicable to each line individually:

- the route of the line is completely separate from the city traffic. Exceptions are level crossings with roads for branches to the depot and connections to the railway;

- Subway lines usually have underground sections and stations. If the line is completely over ground, more than 50% of the total length should be within the boundaries of the urban area;

- The lines are completely separate from the lines of conventional Railways used for passenger and freight traffic;

- Rolling stock consists of no less than two cars in the train; a - line metro trains are not used by other types of rail transportation (even partially), but can tolerate partial use of the path train lines are easily rail systems like Light rail. Rotterdam, For Example. At the same time, the term "metro" does not include sections-branches that are used only by trains of light rail systems;

- If a line has routed traffic and is used in more than half of the departures of trains by other lines (Light rail or tram), which subsequently have an intersection with road traffic or cross on tram tracks, it belongs to type No. 3 (Easy rail). Frankfurt am main, for example;

- If the line has route traffic and is used in more than half of the train departures by suburban lines, which later pass into the status of a regular railway or RER, it belongs to type No. 2 (Urban and suburban Railways). Like Valencia;

 - line can be called with the term "Metro", if the average distance between stations is less than 3.2 km away. All the other systems belong to type 2 (regional metro);

- The interval between metro trains, as a mass mode of transport, in the Central sections of the lines should not exceed 15 minutes;

- Rolling stock always operates on electric power and without the use of separate locomotives.

3.2 Urban and Suburban Railways

    The following parameters are defined for the Urban and suburban Railways group and are applicable to each line individually:

- The lines share all or part of the infrastructure with conventional Railways, level crossings and combined stations.

- the line can be completely isolated from other Railways, but this type is characterized by intersections with at the same level with the street road network and the use of rolling stock adapted to conventional Railways.

- Rolling stock unlike types 3 and 4 always uses high platforms.

- lines connecting the city center to the airport, cannot refer to the term "metro" if the rolling stock is adapted for the carriage of Luggage, has increased comfort and excellent fare system. Thus, these lines essentially serve the transportation of passengers at the airport and does not apply to the term "metro", providing communication between different areas of the city.

3.3  Light rail transport 

    The following parameters are defined for the Light Rail group:

- The lines of this type of transport are isolated from General-purpose Railways, as the rolling stock has smaller dimensions and is not adapted for traffic on common tracks.

- The organization of movement similar to metro line

- Light rail line often have crossing traffic, and are used to route traffic.

- The platforms in most cases have a low level for Pre-Metro/Speed Tram and high level for Light rail.

3.4 Other types of rail transport

   For a group of this type, the description is fully consistent with the above in the Chapter "description of systems".

   General note to this Chapter.

   Due to the lack of a clear classification in the world practice, there are often cases when a system or part of a system is called in accordance with the desire of the developer, the operator company or the official authorities to improve the status of the project or the region as a whole. This is the right strategy to attract investment, but statistically speaking, it is wrong.

4.  Key Parameters of the Systems

4.1 Length of lines

   Statistical data on the length of the system lines and the number of stations very often contain errors and inaccurate data. Information sources and journalists, often without clear criteria, indicate distorted data in their articles. In some cases, the media, government officials and operator companies refer to the length of the lines, which differs from the operational length used in passenger traffic, in statistical reports for the purpose of declaring large figures. For example, the summary figure also takes into account the length of the tracks to the depot, connecting technological branches between the lines or building blocks for future extensions of the lines. These sections are not used in passenger traffic and the length of each can reach 3 kilometers. It is therefore essential to divide the length of the lines into two terms "length of lines for passenger traffic (in double-track calculation)" and "Total construction length of the line (in double-track calculation)". If the second term is not applicable and the line has single-track sections, it can be called as "the Total construction length of the tracks of the line".

   Calculation of length of lines double track in terms of passenger movements should be carried out from the beginning of the impasse in which the train starts moving in passenger traffic to the end point of the deadlock at the other end of the line. If the train arriving at the terminal station then begins to move in the opposite direction, the starting point of the line is the construction end of the station. Measurement error of +/- 10 m. does not matter, as statistics indicated figures in kilometers and hundreds of meters. However, accurate data are usually available in the construction documentation of the line project. Some end stations may be located within the depot and the length calculation must start from a dead end at the station itself. For example, Bakmil station in Baku.

   The general principles for determining the length of the lines are shown in figures 1 and 2

   A number of lines can run parallel to each other in a certain area. In this case, if the tracks are located in the same construction space (three or four way tunnel, overpass, embankment, etc.), it is impossible to consider this section for each line separately, i.e. the concept of "joint use" applies. If the lines are parallel, but physically separated (different tunnels, overpasses, etc.), they must be considered separately.

   In some cities the lines have single-track sections (e.g. Seoul, Paris, and Delhi). In these exceptional cases, it is necessary to consider the full length of the single-track "ring" as a full line. To check the numerical data of information sources on the length of open lines, you can use the service Google Earth (or similar services), which has a function of determining the distance. When opening an actual satellite image, it is impossible to determine the exact location of underground lines and stations, but in most cases, you can refer to the previously taken images of the place in question. If you have at least a superficial knowledge of the stages of construction of underground structures, it is possible without much effort to determine the surface areas on which earthworks were performed and exits to the surface were built. Error in determining the exact coordinates of the station (maximum 100-150 meters) can occur only if the outputs from the station are not symmetrical.

  In New York and London route traffic is very widely used and this fact often does not allow to determine the specific length of the line. In this case, it is necessary to take into account the length of sections used in passenger traffic.

4.2 Number of lines

   You may ask - How can you make a mistake in the number of lines?" and you will be right in this matter. Operators and authorities in different countries have a different system for marking lines on schemes. All the confusion arises on the lines with route traffic. For example, in Bilbao, the two metro lines L1 and L2 in the city center share a common path and are further divided into 2 different routes.  If you look at the scheme of the Hangzhou metro, the subway line No. 1 has a branching into two parts, but the sections have no other designation. The second example is in fact not correct, because. The line has route traffic and must be divided into routes L1A and L1B (or L1 and L1b).

   Sometimes the main metro lines have branches with one or more stations. If such a branch shares traffic with the main line, it should not be considered as a separate line only if the branch has more than one station. If the branch has an independent movement (Tehran, Taipei, Yerevan), these are different lines.

4.3 Number of stations

    Errors are also very common in the statistics on the number of stations. In some systems, stations are counted by the number of individual names, i.e., stations are counted as one station. In other cases, a transfer station built for two lines and having a single space (4 tracks) is considered as two stations. Sometimes in the statistics, especially on the subway of China, get not yet open stations that were not completed at the time of opening lines. These facts indicate a desire to multiply and embellish the overall statistics.

   Criteria for relevance of statistical data:

1) Stations operating or temporarily closed for reconstruction (no more than two to three years) are taken into Account);

2) Stations with one platform of one direction are considered as one station;

3) Closed and dismantled stations are not taken into account;

4) Unfinished or temporarily open stations on new lines are not taken into account;

5) Stations whose platforms are built at a small distance from each other, but having one output and name, must be considered as one station;

6) Three-Way stations located before the branching of the line into two routes are considered as one station;

7) Transfer Stations on lines that intersect at an angle to each other are always considered separately and not by name;

8) Transfer Stations, the lines of which are parallel, are considered as one only if they have one building volume (4 ways), or structurally platforms are located on 2 vertical levels (one station above the other).These transfer stations usually share a common Concourse and Ticket hall (see figure 4). All other cases are considered as two independent stations.

    One example is not the correct account of the number of stations is the station "Kuntsevskaya" Moscow metro. It is a ground station that formerly belonged to the light blue metro line 4. Further, an additional track with a platform for trains of the dark blue line No. 3 was built on the side. During the reconstruction, one of the light blue line tracks was integrated for the dark blue line trains, and the second track became a dead end. As a result, 2 different stations appeared on the schemes of metro lines, but in fact the station is actually one with three ways.

   The general principles for determining the quantity of the stations in parallel lines are shown in figure 3

   The same principles as shown in figure 3 apply to underground stations, but with a few limitations - if the second station was build in parallel as separate structure (communication with the first station only through the transfer corridor), then it must be considered as separate station. Examples for underground and ground stations are shown in figure 4.

   All stations in the transfer hub having different directions (not in parallel) always considered as different stations.