A metro refers to an urban rail transportation system that operates predominantly underground, also known as ‘subway’ or 'underground'. Historically, the first metro in the world was launched in the United Kingdom in 1863, and this new high-capacity mode of transportation effectively alleviated the problem of traffic congestion caused by the population explosion due to the development of urbanization at that time.^[1] The invention of the metro enabled people to commute more quickly and conveniently through crowded cities. With the gradual popularization of the metro, the development of the metro system in Chengdu has been gradually improved.

The Chengdu Metro is a metro system serving Chengdu City in Sichuan Province, China. The first line of Chengdu Metro, namely Chengdu Metro Line 1, was put into operation in September 2010.^[2] As of December 2024, the Chengdu Metro system operates 14 lines, 383 stations, and has a total route length of 631.01 kilometers. As of September 2021, the Chengdu Metro accounted for over 50% of passenger trips, making it the primary transportation mode for Chengdu residents.^[3]

Before the advent of the metro, residents mainly relied on walking and horse-drawn carriages. However, these traditional modes were limited to short distances and low capacity, failing to meet growing urban mobility demands. The advantages of metro system compared to traditional modes of transportation include:

1. Metros are mostly built underground, which saves land space to the greatest extent possible and improves the Efficiency of Land Use.

2. Metros provide High Capacity to effectively alleviate traffic congestion due to the rapid increase in population density.

3. Metros are Reliable and Punctual because they are rarely affected by weather or other transportation modes.

However, the disadvantages of metros are obvious: high construction and maintenance costs and long construction periods. Moreover, for some small and medium-sized cities with small flow of people, the construction of metro not only cannot realize profitability, but also increase the debt pressure.^[4]

The Metro is a pioneer of urban rapid transit. It uses electric traction combined with steel-wheel-on-rail guidance to ensure efficient and eco-friendly operations. The high capacity of the metro well meets the demand of high passenger flow in big cities, which can reach one-way at 30,000 passengers/hour and up to 60,000 to 80,000 passengers/hour. The maximum speed can reach 120km/h, travel speed up to 60km/h or more. It can be 3 to 8 knots formation group, the minimum interval of vehicle operation can be less than 1.5min. Also, the drive mode has direct current motors, alternating current motors, linear motors and so on.^[5]

Chengdu Metro’s signaling systems adopt the moving-block CBTC (Communication-Based Train Control) system,^[6] which utilizes wireless transmission technology and is equipped with automatic train monitoring, automatic train guarding, automatic train operation and other functions, which can ensure the safety of train operation, prevent rear-end collision and conflict, improve the operation efficiency (shorten the interval between trains under the premise of guaranteeing safety), and realize the informatization and automation of train operation.^[7]

The 3D inspection system, firstly developed by the Chengdu Metro, was successfully implemented in May 2020 in Chengdu Metro,^[8]which provides more high-quality and efficient technology and service for metro construction. The system is installed on the rail vehicles, and by adopting advanced technologies such as high-precision sensing, 3D laser scanning, and combined navigation, it is able to follow the vehicles and take photos, thus quickly and accurately locking the problematic points of tunnels. Tunnel 3D inspection system focuses on solving the current traditional tunnel inspection time-consuming, easy to miss, low efficiency and other difficult problems, this system has real-time, dynamic, all-round, high-precision, high-efficiency inspection characteristics. The technology significantly improves the software and hardware facilities, builds a multi-visual omni-directional measurement system, which makes the image shooting of various dimensions more accurate.^[9]

While technological advancements have driven the Metro system capabilities, policy frameworks play an equally critical role in shaping their development trajectory.

2005 – 2012 Phase I Construction:

• In 2005, the China NDRC (National Development and Reform Commission) officially approved the Chengdu Urban Rapid Transit Construction Plan, also known as the first phase of the Chengdu Metro Construction Plan. It issued the ‘Approval of the Feasibility Study Report on the First Phase of Chengdu Metro Line 1’, which formally approved and agreed to build the first phase of the Chengdu Metro Line 1 project; Line 1 is the north-south line in the Chengdu meter-shaped backbone network.^[10]

• In 2007, the NDRC issued the ‘Reply to the Feasibility Study Report of Chengdu Metro Line 2 Phase I Project',^[11]formally approving the construction of Chengdu Metro Line 2 Phase I Project, as Line 2 is the northwestern-southeastern line in the Chengdu Metro meter-shaped backbone network.

• In 2012, the NDRC approved the construction of the first phase of Chengdu Metro Line 3 and Line 4 while Line 3 is the northeast-southwest line of the Chengdu Metro's meter-shaped backbone network and Line 4 is the east-west line of the Chengdu Metro's meter-shaped backbone network.^[12]

The first phase of construction aims to expand the service scope of rail transit, optimize the urban passenger transport structure, and accelerate the development of new urban areas in the south of Chengdu. The successful implementation of Phase I established the meter-shaped backbone network, setting the stage for subsequent expansions.

2013 – 2020 Phase II Construction:

• In 2013, the NDRC approved the Chengdu Urban Rail Transit Recent Construction Plan, also known as the Chengdu Metro Phase II Construction Plan.

• By 2020, the construction had completed with the Line 1 Phase III project, the Line 3 Phase II and Phase III projects, the Line 4 Phase II project, the Line 5 Phase I and Phase II projects, the Line 6 Phase I project, the Line 7 project, and the Line 10 Phase I project, as well as the addition of a New Airport Line (Line 18), line Chengdu South Railway Station to Chengdu New Airport Station.

The second phase of the construction is aimed at building a fast link between Chengdu Shuang Liu International Airport, Tian Fu International Airport and Tian Fu New Station, creating conditions for the 'two fields in one' mode of operation, and forming a fast channel covering Shuang Liu and Wen Jiang to the west from the core area of Tian Fu New Area, in order to promote the seamless connection between various means of transportation such as aviation, railroad, highway and metro, and create an international comprehensive transportation hub in Chengdu.^[13] The completion of Phase II marked Chengdu's transition from a city metro to a regional transportation hub.

2020- 2024 Phase III Construction and Phase IV Optimization:

The construction of Line 8 Phase I, Line 9 Phase I, Line 10 Phase II, Line 11 Phase I and Line 17 Phase I, a total of five projects with a total length of 124.2 kilometers. By 2020, a rail transit network of 13 lines with a total length of 508 km will be formed.

From 2020 to 2024, eight projects will be constructed, including Line 8 Phase II, Line 10 Phase III, Line 13 Phase I, Line 17 Phase II, Line 18 Phase III, Line 19 Phase II, Line 27 Phase I, and Line 30 Phase I, with a total length of 176.65 kilometers. After the completion of the project, Chengdu will form a total length of about 692 kilometers of rail transit network.

The third phase of construction aims to coordinate rail transit with surrounding land development, optimize land use around stations, and gradually realize the TOD (Transit-Oriented Development) mode.^[14]The fourth phase of construction aims to optimize the laying of lines and station spacing outside the central city to achieve the planned function of rapid access. In addition, it will optimize the comprehensive traffic connection and improve the overall efficiency and attractiveness of public transportation.^[15]The phased expansion reflects Chengdu's evolving urban strategy: from solving traffic congestion to shaping metropolitan spatial patterns.

Chengdu has an important strategic position as the economic and cultural center of Southwest China. Chengdu was the first city in central and western China to open an operational metro line. As a major hub in central and western China, Chengdu faces challenges such as population growth, urban expansion and traffic congestion. Therefore, the metro system was developed to alleviate traffic pressure, while also supporting the economic development of the city, enhancing its competitiveness, and promoting the integration of the 'Chengdu-Chongqing Economic Circle'. This explosive growth validated the metro's role not just as transportation infrastructure, but as an economic accelerant. Having established the historical and policy context, following part will quantitatively examine ridership patterns through logistic growth modeling.

In the quantitative analysis part, this article will use the following three-parameter logistic function to map the Chengdu Metro growth lifecycle.

S(t) = Smax/[1+exp(-b(t-ti)]

By calibrating the model against empirical data, this part tries to derive critical parameters that capture saturation points, growth rates, and inflection dynamics - essentially decoding the DNA of development patterns of Chengdu Metro system.

In the Chengdu Metro case study, this study selects the total annual passenger volume (100,000 trips per year) as the variable of the S-curve. This approach may allow to quantitatively track the lifecycle evolution of metro ridership.

• S(t): Market size at a given time t. In this study, it refers to the ‘total annual passenger volume (100,000 trips/year)’.

• t: Time, 'year' in this study.

• ti: Inflection year, it is the ‘the year when 50% of  is reached’.

• Smax: Maximum saturation level, referring to the ‘maximum passenger capacity under stable conditions’ in this case.

• b: Growth rate’, the engine driving adoption velocity (to be estimated).

This study collected data on time  and total annual passenger volume  from 2011 to 2024, which was mainly sourced from CEIC Data and Chengdu Rail Transit, ^[16][17][18] as shown in Table 1.

Table 1 Actual total annual passenger volume for Chengdu Metro from 2011 to 2024

According to a January 2nd, 2025 news, Chengdu Metro network set a new historical record for daily passenger volume on December 31, 2024, reaching 8.3218 million passenger trips in a single day.^[17]This value is adopted as the maximum passenger capacity under saturation conditions. It is estimated the annual passenger volume as:

Smax=8.3128*365=303,745.7(ten thousand/year)

The inflection year  is the year when passenger volume reaches, which is probably 151,873 (100,000 trips/year). From the data:

· 2020: 139,943 (100,000 trips/year)

· 2021: 156,193 (100,000 trips/year)

Since the ridership surpassed 151,873 million in 2021, the inflection year ti is approximately 2021.

With the purpose to estimate b, the non-linear regression based on the least squares method is selected. The goal is to fit the model in a way that minimizes the difference between predicted and actual total annual passenger volume data.The final estimated parameters results are:

• Smax=252,878 (100,000 trips per year)

• ti= 2019.25 (inflection year)

• b = 0.37 (growth rate)

The scatter plot and line chart were created in Excel using actual and predicted values as shown in Figure 1. From the chart, the horizontal axis represents the data years from 2011 to 2024, while the vertical axis corresponds to passenger volume.

The graph shows a typical S-curve growth pattern with below features:

• Slow growth in the initial phase,

• Rapid expansion in the middle phase,

• Gradual stabilization as it approaches the saturation level Smax

The overall trend shows a continuous rise in ridership over time, with a particularly noticeable acceleration after 2015. This growth may be attributed to factors such as steady population influx, opening of new metro lines or extended lines, and the increasing dominance of metro systems in public transportation.

Comparing the linear fit with the Logistic model, it becomes clear that the linear trend line underestimates ridership growth after 2019, failing to capture the sharp upward trend. In contrast, the Logistic curve aligns much more closely with the blue scatter points, demonstrating that over a longer time span, S-curve growth more accurately describes the diffusion of metro ridership.

The R² value of approximately 0.9749 indicates that the model explains about 97% of the variation in passenger volume, confirming a relative good fit. This further supports the idea that the total annual metro passenger volume follows an S-shaped diffusion pattern, with steady early growth, rapid mid-phase expansion, and eventual saturation.

From the shape of the curve, the predicted trend continues to rise steadily after 2020, but the growth rate begins to slow down, indicating that ridership may gradually approach a saturation level and enter a more stable development phase.

This insight has important implications for transportation planning and resource allocation:

• It helps to assess future demand for new metro lines or capacity adjustments.

• It provides a reference for government and operators in areas such as fare policies and service optimization.

Overall, the chart illustrates that the ridership of Chengdu Metro - from initial growth to expansion and eventual stabilization. The high R² value confirms the reliability of the Logistic model's predictions, making it a valuable tool for guiding future development.

The actual ridership, around 78,000 (100,000 trips/year) surpassed the linear prediction, likely due to ‘Rail + TOD’ Development Strategy. In 2017, Chengdu launched its TOD policy, which significantly enhanced land value around metro stations through the ‘137 Development Model’:^[19]

i. Core Zone (100-meter radius):

• Focus: Commercial and service facilities.

• Goal: Develop highly visible urban landmarks.

ii. Sub-Core Zone (300-meter radius):

• Focus: Office spaces, hotels, and public service facilities.

• Goal: Create dynamic and diverse urban scenes.

iii. Non-Core Zone (700-meter radius):

• Focus: Residential areas, parks, and ecological spaces.

• Goal: Foster a balanced living environment.

In 2020, the actual ridership was about 500,000 less than predicted, likely reflecting the impact of COVID-19 on ridership. However, the post-2020 recovery saw a significant increase in total passenger volume growth, which could be directly linked to the operation of the first 10 TOD projects (such as Lu Xiao Station and Shuang Feng Qiao Station) as well as the line opening of Tian Fu International Airport, which triggered a significant surge in commuting demand.

This validates the idea of a positive cycle that metro ridership sustains the metro system, while the metro system promotes urban development. In 2021, the National Development and Reform Commission's instruction of ‘Multilevel Rail Transit Plan for the Chengdu-Chongqing Economic Circle’ emphasized the construction of a one-hour commuting circle, prompting the extension of Chengdu's Metro network into regional rail lines.^[20]The opening of Line 18 (extended to Jian Yang South Station in 2023) and Line 27 (connecting Xin Du to Tian Fu Airport) not only serves commuter traffic but also facilitates industrial population flow, reinforcing the sharp increase in ridership after 2022.

Since 2022, Since 2022, China has implemented nationwide restrictions on fuel vehicle licenses while incentivizing new energy vehicles. This policy shift unexpectedly boosted metro demands. The non-linear model's forecast now shows a more rapid rise in ridership, which requires attention to TOD development progress and new energy vehicle penetration rates, as these variables will influence the growth trend.

1. ↑ Lin, Dong; Broere, Wout; Cui, Jianqiang (2022-07). "Metro systems and urban development: Impacts and implications". Tunnelling and Underground Space Technology. 125: 104509. doi:10.1016/j.tust.2022.104509. ISSN 0886-7798. {{cite journal}}: Check date values in: |date= (help)
2. ↑ Qiu, Zhiyue; Wen, Shirui; Yuan, Hong; Liu, Ziyi; Wei, Yao; Yanling, Siqi; Dai, Runlong; Li, Xiang; Gu, Yuxin (2025-02-23). "Evaluation Methods for the Human–Land Coupling Coordination Relationship in a Metro Station Area: A Case Study of Chengdu Metro Line 1". ISPRS International Journal of Geo-Information. 14 (3): 102. doi:10.3390/ijgi14030102. ISSN 2220-9964.
3. ↑ Wang, Sicheng; Noland, Robert B. (2021-01). "Variation in ride-hailing trips in Chengdu, China". Transportation Research Part D: Transport and Environment. 90: 102596. doi:10.1016/j.trd.2020.102596. ISSN 1361-9209. {{cite journal}}: Check date values in: |date= (help)
4. ↑ Zhang, Yajing; Jin, Weijian; Yuan, Jingfeng (2023-09-22). "Policy Perspective on Governmental Implicit Debt Risks of Urban Rail Transit PPP Projects in China: A Grounded Theory Approach". Sustainability. 15 (19): 14078. doi:10.3390/su151914078. ISSN 2071-1050.
5. ↑ Zhuo, Banglin; Ye, Zhihao; Wei, Kun (2024-12-03), "Review on the Key Technologies of Linear Induction Motors for the Rail Transit", Lecture Notes in Electrical Engineering, Singapore: Springer Nature Singapore, pp. 295–310, ISBN 978-981-97-8831-6, retrieved 2025-03-08
6. ↑ He, Yijuan; Lv, Jidong; Tang, Tao (2022-08-18). "Communication-Based Train Control with Dynamic Headway Based on Trajectory Prediction". Actuators. 11 (8): 237. doi:10.3390/act11080237. ISSN 2076-0825.
7. ↑ Qiu, Chen; Chen, Tan; Lu, Shaofeng; Wang, Haifeng (2022-05). "A Safety-Oriented Dynamic Moving Block Train Control System Based on Train-to-Train Communication". IEEE Intelligent Transportation Systems Magazine. 14 (3): 175–187. doi:10.1109/mits.2021.3049369. ISSN 1939-1390. {{cite journal}}: Check date values in: |date= (help)
8. ↑ Wang, Xuejian; Wang, Jiayuan; Zhang, Yuyu (2023-09-12). "Research on 3D Visualization of Real Scene in Subway Engineering Based on 3D Model". Buildings. 13 (9): 2317. doi:10.3390/buildings13092317. ISSN 2075-5309.
9. ↑ 邵, 天元 (2022). "Synergistic Effect Evaluation of Rail Transit Stations Areas—Taking Central Rail Transit Stations in Nanjing as an Example". Open Journal of Transportation Technologies. 11 (04): 331–337. doi:10.12677/ojtt.2022.114033. ISSN 2326-3431.
10. ↑ Abramson, Daniel; Qi, Yu (2011-09-01). ""Urban-Rural Integration" in the Earthquake Zone: Sichuan's Post-Disaster Reconstruction and the Expansion of the Chengdu Metropole". Pacific Affairs. 84 (3): 495–523. doi:10.5509/2011843495. ISSN 0030-851X.
11. ↑ Peng, Zhong-Ren; Lu, Kaifa; Jin, Mengyi; Zhu, Xinghang; D. Landis, John (2022-12-08), "China's metro explosion: lessons from Chinas big four cities", Megaprojects for Megacities, Edward Elgar Publishing, pp. 191–235, ISBN 978-1-80392-063-4, retrieved 2025-03-08
12. ↑ Yin, Yong; Chen, Jinqu; Peng, Qiyuan (2022). "Measures of Accessibility on an Urban Rail Transit System from the Perspective of Passengers' Travels". SSRN Electronic Journal. doi:10.2139/ssrn.4100436. ISSN 1556-5068.
13. ↑ "四川省发展和改革委员会". fgw.sc.gov.cn. Retrieved 2025-03-08.
14. ↑ "成都规划39条城市轨道交通线路 总规模约2370公里". jtt.sc.gov.cn. Retrieved 2025-03-08.
15. ↑ "中国共产党中央委员会 中华人民共和国全国人民代表大会常务委员会 中华人民共和国国务院 中国共产党中央军事委员会——关于建立伟大的领袖和导师毛泽东主席纪念堂的决定". Chinese Science Bulletin. 21 (Z1): 433–433. 1976-07-01. doi:10.1360/csb1976-21-z1-433-x. ISSN 0023-074X.
16. ↑ "中经数据-专注中国宏观经济统计数据30年". ceidata.cei.cn. Retrieved 2025-03-08.
17. ↑ ^{a b} "中国新闻网| 832.18万！成都轨道交通客流再创新高-成都轨道集团". www.chengdurail.com. Retrieved 2025-03-08.
18. ↑ "成都地铁". www.720yun.com. Retrieved 2025-03-08.
19. ↑ "四川发布：今日，领证！-成都轨道集团". www.chengdurail.com. Retrieved 2025-03-08.
20. ↑ 陈, 国庆 (2023). "Research on the Coordinated Development of Sports Industry and Regional Economy in Chengdu-Chongqing Double-City Economic Circle". Advances in Social Sciences. 12 (04): 1735–1746. doi:10.12677/ass.2023.124236. ISSN 2169-2556.