Through service

A through service is a public transport operation that continues from one railway line, route, or network to another without requiring passengers to change vehicles.^[1] The arrangement provides a single-seat ride across operational boundaries and is distinct from a direct service, which may stay within one line or operator. The practice appears in rail and bus systems and can involve different infrastructure owners, power systems, or line designations.

Through services take several forms. A vehicle may traverse different lines or infrastructures, such as a mainline railway and an urban rapid transit line.^[2] A service may keep passengers on board while changing its public designation en route. Operators may also combine two previously separate routes to create a single A–B–C journey.^[3][4]

Transport agencies use through services to expand network reach and reduce passenger interchange. The approach promises shorter overall trips and simpler wayfinding for riders.^[5] Operators may also improve asset utilization by scheduling vehicles and crews across what would otherwise be two terminal turns.

Through services can dramatically increase network capacity by eliminating the operational inefficiencies of stub-end terminals. Traditional terminal stations require extended dwell times of 12–22 minutes for crew changes, train cleaning, and turnaround procedures, compared to 60–90 second stops at through-stations.^[6] Through-running can reduce station dwell times to as little as 3 minutes, allowing each track to handle significantly more trains per hour.^[7]

The elimination of conflicting movements in station "throats"—where departing trains must cross the paths of arriving trains—removes a critical bottleneck that often limits overall system capacity more than the number of available platforms.^[8]

Through services can generate significant economic benefits beyond improved passenger convenience. By creating unified regional networks, they expand the geographic range of job opportunities available to residents and talent pools accessible to employers. Studies suggest that every dollar invested in rail infrastructure generates approximately $2.50 in broader economic activity.^[9]

The enhanced connectivity often catalyzes transit-oriented development (TOD) around stations, concentrating dense, mixed-use, walkable development within walking distance of high-frequency service. This development pattern can help address housing affordability while reducing automobile dependence.^[10] However, transit improvements can also contribute to gentrification and displacement of existing communities as property values rise around improved stations.^[11]

Significant constraints accompany the benefits. Vehicles must be compatible with all lines they traverse, including track gauge, platform interface, loading gauge, tunnel and bridge clearances, power supply, and signaling.^[12][13]

Modern through services increasingly deploy multi-system rolling stock capable of operating across different electrification standards. The Paris RER system pioneered dual-voltage operation in the 1960s with specialized rolling stock like the MI 79, designed to operate on both SNCF's 25 kV AC system and RATP's 1,500 V DC network.^[14]

Implementation often requires unprecedented coordination between different operators, jurisdictions, and regulatory frameworks. Issues include harmonizing labor agreements across different union contracts, integrating fare systems, and establishing unified service standards. These institutional barriers often prove more challenging than technical compatibility issues.^[15]

Reliability can be more fragile because a delay on one segment can propagate to the rest of the through-routed pattern. Greater Tokyo's extensive use of through services illustrates both the connectivity advantages and the systemwide ripple effects from even minor incidents.^[16] Coordinating fares, schedules, crew qualifications, and passenger information across operators also adds complexity.

A train providing a through service is often called a through train^[17] and may also be described as a run-through service or train,^[18] or an interline service in some North American practice.^[19][20] In broader transit scheduling, interlining can also mean the coordinated assignment of vehicles across different routes to improve resource use, which may or may not permit a continuous passenger ride. This article uses through service for the passenger-facing, continuous operation across lines or operators.

Through services often depend on trackage rights or comparable operating agreements. Trackage rights describe the commercial permission to use others' tracks. Through service is the passenger-facing result on a combined route. The concepts are related but not identical.

Operational feasibility hinges on infrastructure and rolling stock compatibility, including power systems and signaling.^[21] In practice, many networks design common standards or deploy dual-system or multi-system equipment.^[22] Variable-gauge technology exists but is uncommon for frequent passenger through services due to cost and time penalties at gauge-changing installations.

From a user perspective, through operation is sometimes invisible when lines are branded as one network. In other cases, a change of train number or operator occurs mid-journey while passengers remain on board.

Melbourne: Many services of Metro Trains Melbourne through-run between the Werribee and Williamstown lines and the Frankston line via Flinders Street. The Cross City group was formed in 2011 by combining these lines, with all services running direct to and from Flinders Street and most continuing through to the other side. From 2025, when the Metro Tunnel opens, the Frankston line will return to the City Loop, while the Werribee and Williamstown lines will instead through-run with the Sandringham line.

Perth: Transperth services commonly through-run between the Yanchep line and Mandurah line and between the Midland and Fremantle lines.

Brisbane: Northside and Southside suburban lines through-run via Roma Street.

In Toronto, GO Transit's Lakeshore East line and Lakeshore West line often interline across the Union Station Rail Corridor, creating continuous rides through downtown. The Maple Leaf between Toronto and New York City operates as one passenger service with a change of operator at the international boundary.^[23] Via Rail crews run Toronto to Niagara Falls and Amtrak crews run to New York. Passengers usually alight at the border for inspection procedures.

Train numbers in mainland China reflect line direction. Up trains toward Beijing carry even numbers and down trains carry odd numbers. A long-distance service that changes directional classification mid-route changes its train number while passengers remain aboard. For example, a Guangzhou to Lhasa service operates Guangzhou–Zhengzhou as Z264, then continues as Z265 from Zhengzhou to Lhasa.

Several urban metro systems operate through services across nominal line boundaries. Use of Chinese terms follows accessibility guidance. The commonly used term for through operation is 贯通运营. Non-Latin scripts are not bolded or italicized in accordance with formatting practice.

Beijing Subway: Line 1 and Batong line began through service on 29 August 2021;^[24] most Line 4 trains continue into the Daxing line;^[25] Line 9 and the Fangshan line added limited through service in January 2023.^[26]

Kunming Metro: Line 1 and Line 2 operated through services at South Ring Road before later phases split operations.

Nanjing Metro: Some Line S1 trains continue into Line S7.

Zhengzhou Metro: Selected Line 2 trains continue onto the Chengjiao Line.

Guangzhou Metro: The Line 3 main and airport branches operate selected through trips between Haibang and Airport North in addition to standard branch service patterns.

Chongqing Rail Transit: Express services on Line 4 through-run with the Loop Line and later extended to Line 5.

Dalian Metro: Line 3 branch and Line 13 operate through services.

Suzhou Rail Transit: Through service between Line 3 and Line 11 began in 2023,^[27] with additional limited through trains between Line 4 and Line 7 during weekday peaks from 28 August 2025.^[28]

Paris Réseau express régional uses through services across different owners. RER A trains run over SNCF infrastructure in the west and RATP in the east. RER B similarly spans SNCF in the north and RATP in the south. Cross-network operation led to jointly specified rolling stock such as the MI 79.^[29]

In Germany, such services are commonly called Durchbindung. The Munich S-Bahn provides a notable example, linking together 12 pre-existing suburban branch lines with a central tunnel that opened in the 1970s.^[30] The system integrated with the U-Bahn metro network to create interchange points and unified fare structure, demonstrating the network effects possible with coordinated through services.

In the MTR Light Rail, vehicles on routes 614P and 615P continue onto each other's alignments at their termini in Siu Hong and Tuen Mun Ferry Pier. On the heavy metro, the Kwun Tong line can be through-routed during disruptions to part of the Tsuen Wan line or the Tseung Kwan O line.

Through services 直通運転 (chokutsū unten) are widespread on Japanese urban railways. Operators use them to extend reach and reduce transfers, for example on airport expresses linking Narita and Haneda over tracks of Keikyu, Toei, Keisei, and the Hokuso Railway. Congested central segments can magnify delays into suburban partners, which has drawn policy and operations attention.^[31]

The Seoul Metropolitan Subway operates through services between city subway lines and Korail suburban lines, including the combined Suin–Bundang Line.

Russia runs scheduled international through services, including Moscow–Beijing via the Trans-Siberian Railway and Trans-Manchurian Railway, Moscow–Ulaanbaatar via the Trans-Mongolian Railway, and other routes.

Most of Great Britain's mainline railway is owned and operated by Network Rail. On National Rail a through train is a journey that does not require passengers to change trains.^[17] Examples include long-distance services that traverse multiple named Network Rail lines. There are also cross-owner cases.

The Thameslink network represents one of the UK's most comprehensive implementations of through-running. The system connects Bedford, Cambridge, and Peterborough in the north to Brighton, Horsham, and other southern destinations via a core tunnel through central London.^[32]

The network demonstrates how through services can transform fragmented commuter operations into an integrated regional rail system. The original concept dates to Victorian times, when passenger services operated through the Snow Hill tunnel until World War I. The modern Thameslink launched in 1988 after the tunnel's reopening, immediately experiencing demand that exceeded projections by 300%.^[33]

The Thameslink Programme, completed in 2018 at a cost of £5.5 billion, expanded the system's capacity and destinations. The network now carries more than 28,000 passengers during morning peak periods and operates some sections 24 hours daily.^[34]

Thameslink Class 700 rolling stock operates on both 25 kV AC overhead catenary systems north of Farringdon and 750V DC third rail systems to the south, demonstrating the technical feasibility of multi-system compatibility. The network integrates multiple pre-existing railway lines including the Brighton Main Line, Midland Main Line, and East Coast Main Line into unified service patterns.^[35]

The Bakerloo line operates on London Underground infrastructure and onto the Watford DC line owned by Network Rail up to Harrow & Wealdstone. The District line operates to Richmond over the North London line. Chiltern Railways services between London Marylebone and Amersham use London Underground tracks between Amersham and Harrow-on-the-Hill. The Elizabeth line links Shenfield to Heathrow Airport over Network Rail in the east, Crossrail's central section, and the Great Western Main Line plus the Heathrow spur with in-motion signaling transitions. The Sheffield Supertram tram-train service runs between the tram network and mainline rail.

North American usage often calls through-routed operations interlining. APTA defines interlining as the interchange of passengers between one or more bus lines, rail transit lines, or railroads and the transfer of vehicles or trains between routes to improve assignments.^[19][20] Examples include:

Portland, Oregon: MAX Light Rail's Yellow Line and Orange Line share tracks on the Portland Transit Mall.^[36]

Westchester–New York City: Metro-North Railroad trains run through to New York City territory with equipment that can switch between third-rail electric and diesel power.

Chicago: The Purple Line Express continues onto Brown Line tracks to the Loop at weekday peaks.

In the Northeastern United States, several major proposals seek to convert stub-end terminals into through-running hubs, representing a fundamental shift from traditional "commuter rail" to "regional rail" service patterns.

At New York Penn Station, advocates have outlined approaches to link NJ Transit and the Long Island Rail Road for cross-region commutes.^[37] The Gateway Program would add new Hudson River tunnels that proponents view as a precondition for such integration.

The proposal faces complex technical barriers rooted in a century of separate development. NJ Transit uses overhead catenary power systems (12 kV 25 Hz AC/25 kV 60 Hz AC), while the Long Island Rail Road operates on 750V DC under-running third rail. Integration would require either a new fleet of dual-system trains similar to Metro-North Railroad's M8 units, or systematic electrification standardization across both networks.^[38]

The operational model change from terminal to through-running could accommodate doubled trans-Hudson capacity from Gateway's new tunnels within Penn Station's existing footprint, potentially avoiding the proposed $16.7 billion Penn Station South expansion.^[39] However, this requires resolving complex labor agreements across multiple union locals and different state agencies, as no unified regional authority exists to coordinate implementation.^[40]

The proposed North–South Rail Link would heal a century-old division in Boston's rail network, connecting North Station (serving New Hampshire, Maine, and northern suburbs) with South Station (serving southern and western lines) via a downtown tunnel.^[41]

The 2018 Massachusetts Department of Transportation feasibility study estimated costs between $12.3–21.5 billion for various tunnel alignments, though alternative analyses suggest significantly lower figures ($3.8–5.9 billion).^[42][43]

The project faces unique constraints including the requirement for full network electrification (diesel locomotives cannot operate safely in long tunnels), proximity to existing Big Dig infrastructure, and political skepticism following that project's cost overruns. The four-track Central Artery alignment would be the only option connecting the Fairmount Line, which serves many of Boston's lower-income communities.^[44]

The Washington metropolitan area has conducted extensive feasibility studies for through service between the Maryland Area Regional Commuter (MARC) and Virginia Railway Express (VRE) systems. A 2020 study by the Metropolitan Washington Council of Governments estimated potential ridership of over 16,000 daily trips by 2030, with the greatest demand between Baltimore, Maryland and Alexandria, Virginia at approximately 11,600 trips per day.

The study found that approximately 13,900 "run-through equivalent" trips already occur daily, with passengers transferring between MARC, VRE, and Metrorail to complete cross-jurisdictional journeys. Implementation would eliminate many of these transfers, with potential to double ridership at L'Enfant Plaza and increase Crystal City ridership by one-third.

Technical challenges include coordinating different power systems (MARC uses both diesel and electric traction, while VRE operates diesel-only), harmonizing crew qualifications across state lines, and managing capacity constraints during construction of major infrastructure projects including the Long Bridge expansion and Union Station renovations. The analysis identified institutional coordination as potentially more challenging than technical compatibility issues.^[45]

Both proposals represent evolution from traditional "commuter rail" focused on peak-hour, downtown-oriented travel to "regional rail" providing frequent, all-day, multi-directional service across entire metropolitan areas.^[46]

On bus systems, through-routing links routes across a central area so riders continue across downtown without changing vehicles. In some operations a bus arriving as one route departs as another while allowing passengers to remain aboard, sometimes with a through fare. In Bournemouth, morebus routes 16 and 17 can be scheduled as a continuous journey through Bournemouth Square.^[47]

Trackage rights

Express train

Intermodal passenger transport