San Sebastián’s ‘Topo’ Doubles Frequency with New Underground Stretch

San Sebastián’s “Topo” suburban rail line, nicknamed the Mole, has taken a significant step toward metro-level service, with a newly opened underground stretch between Altza and Galtzaraborda allowing trains to run roughly twice as often on one of Gipuzkoa’s busiest commuter corridors.

New underground variant removes Altza bottleneck

The new underground variant between Altza, in eastern San Sebastián, and Galtzaraborda, in nearby Errenteria, entered service on July 18 following overnight connection works and final signal testing. Publicly available information indicates that the project replaces the former single-access “dead-end” configuration at Altza with a double-track through tunnel of almost two kilometers.

This new alignment allows trains arriving from Amara, in central San Sebastián, to continue directly toward Pasaia and Irun without reversing, eliminating a time-consuming operational maneuver. The change is described in regional government documentation as a key step in transforming the coastal narrow-gauge line into a higher-capacity urban and metropolitan rail spine.

The underground section also removes surface rail in sensitive residential areas and reduces rail traffic on an elevated viaduct close to homes in Pasaia, a long-debated issue in local planning discussions. The shift underground is being framed in local coverage as both a transport efficiency measure and an urban integration project.

According to recent government program updates, the Altza–Galtzaraborda opening is part of a broader “pasante soterrada,” or underground cross-city link, designed to reshape how trains pass through Donostia-San Sebastián and connect with neighboring municipalities along the Bay of Biscay.

Weekday services rise from 136 to 274 trains

Operational data made public with the launch indicate a sharp increase in weekday services. Reports on the new timetable describe an expansion from 136 to 274 train movements on working days across the affected corridor, effectively doubling overall service.

Frequency is shifting from 15-minute to 7.5-minute headways between Amara and Pasaia in weekday daytime periods, aligning the Topo more closely with metro-style operations. Government communications on the wider Donostia railway program have long cited 7.5-minute intervals as the target standard for the reconfigured network.

The doubled frequency is expected to shorten waiting times for daily commuters traveling between eastern San Sebastián, Pasaia and Errenteria, and to improve connections toward Irun and the French border at Hendaye. Timetable notices from Euskotren for the current summer season already highlight increased services on the Topo lines, with the Altza branch now operating as a full through-service rather than a secondary spur.

Transport analysts following the project note that higher capacity could encourage a modal shift from private cars on short and medium-distance trips within the metropolitan area, especially if the improved frequencies are maintained consistently through peak periods and complemented by coordinated bus and regional rail schedules.

Part of a wider underground cross-city rail project

The Altza–Galtzaraborda variant is one piece of a much larger remodelling of San Sebastián’s urban rail network, centered on the so-called pasante soterrada beneath the city. This cross-city tunnel, around 4.2 kilometers in length, includes three new underground stations and is intended to provide a continuous, grade-separated route for Euskotren services through Donostia.

Recent progress updates from the Basque Government’s mobility department indicate that civil works on the central section are nearing completion, including the new La Concha underground station and the Bentaberri terminal. Systems installation, fire protection, lighting and access equipment are in advanced stages, with rail infrastructure such as catenary supports already in place in much of the tunnel.

According to published planning documents, the cross-city tunnel will eventually allow metro-like frequencies for Euskotren services between Añorga to the west of the city and Pasaia to the east, again based on 7.5-minute intervals on weekdays. Combined with the Altza–Galtzaraborda variant, this is intended to create a continuous high-capacity corridor across the whole urban area.

There are also plans for an interchange hub at Riberas de Loiola, where Euskotren’s narrow-gauge Mole line and Spain’s standard-gauge Renfe services will meet in a single space, simplifying transfers between regional trains and the suburban network.

Urban regeneration and quieter streets along the Topo

Beyond capacity gains, regional and municipal planning texts emphasize the urban regeneration dimension of the Topo’s underground expansion. By taking more of the line below ground, the project is expected to remove several level crossings, reduce noise in densely populated neighborhoods and free surface land for new public spaces.

In central San Sebastián, early urbanization works have begun around streets such as Loiola and San Martín in anticipation of station openings, with designs focused on pedestrian circulation and integration with existing city blocks. Once the underground stations are fully operational, former railway alignments and service yards are slated for redevelopment, including new green areas and improved cycling and walking links.

Along the eastern corridor toward Pasaia and Errenteria, the shift from exposed viaducts and single-track bottlenecks to subterranean double track is intended to address long-standing concerns about visual impact and proximity to housing. Publicly available material on the project suggests that some rail-adjacent plots could eventually be repurposed for housing or community facilities.

Local mobility strategies frame these changes as part of a larger attempt to rebalance space away from private vehicles, encouraging public transport, walking and cycling through better-quality public realms around key stations on the Topo line.

A growing role in cross-border and coastal mobility

The Mole line already plays a central role in cross-border mobility between Spain and France, with Euskotren services extending beyond Irun to Hendaye, where passengers can connect with the French rail network. Rail enthusiasts and transport observers frequently highlight this route as a rare example of frequent commuter-style services crossing a European internal border.

With the latest underground variant delivering more capacity and shorter intervals, regional planning documents suggest that the line is increasingly being positioned as a backbone for sustainable travel along the eastern Gipuzkoa coast. Seasonal timetable adjustments in recent years have already seen extra services laid on for summer tourism flows to and from Donostia-San Sebastián.

Parallel infrastructure projects elsewhere in the Basque Country, including ongoing work on high-speed rail and tunnels on the wider Bilbao–San Sebastián axis, are expected over time to interact with the upgraded Topo network. Publicly accessible summaries of these schemes indicate that improved regional and long-distance connections could funnel more passengers into the metropolitan rail system, reinforcing the strategic importance of the Mole’s new underground segments.

For now, the immediate impact for passengers is straightforward: trains through eastern San Sebastián and into Pasaia and Errenteria are arriving roughly twice as often as before, on a line that is steadily retreating from view beneath the city as it takes on more of the characteristics of a full urban metro.