Metro, Light Rail, and Trams: The Technical Differences Explained

⚡ IN BRIEF

• The 1976 Edmonton LRT – Birth of the “Light Rail” Category: When Edmonton, Canada, opened its first light rail line in 1978, it was the first system to deliberately combine street‑running sections (like a tram) with a dedicated downtown tunnel (like a metro). The term “Light Rail Transit” (LRT) was coined to describe this hybrid, filling a gap between traditional trams and heavy metros.
• Metro (Heavy Rail) – Defined by Segregation: The defining technical characteristic of a metro is its exclusive right‑of‑way – grade‑separated from all other traffic (tunnels, viaducts, or protected surface lines). This enables high speeds (80‑100 km/h), high capacities (30,000‑80,000 passengers per hour per direction), and fully automated operation (GoA 4) on many systems.
• Light Rail (LRT) – The Hybrid: LRT systems combine elements of both: they may have exclusive right‑of‑way (like a metro) in corridors with high demand, but also run on streets (like a tram) in lower‑density areas. Modern LRT vehicles have higher acceleration (1.2‑1.5 m/s²) than trams, can operate in multiple units, and are designed for speeds up to 80 km/h.
• Trams (Streetcar) – The Urban Integrator: Trams are designed to operate in mixed traffic, with short stop spacing (300‑500 m) and low speeds (10‑20 km/h). They have lower floors (300 mm) for street‑level boarding, sharp curve capabilities (minimum radius 15‑25 m), and are optimised for accessibility rather than speed.
• The Tram‑Train Revolution – Blurring the Lines: Emerging systems like the “Karlsruhe Model” (Germany) use dual‑voltage vehicles that can run on city tram tracks (750 V DC) and mainline railway tracks (15 kV AC). This hybrid concept, now defined in the TSI OPE and TSI LOC&PAS, combines the accessibility of trams with the reach of regional rail.

Editor’s Analysis: The Hybrid Future – Why the Categories Are Blurring

The traditional distinctions between metro, light rail, and tram are becoming increasingly irrelevant. The rise of the tram‑train, the proliferation of LRT with downtown tunnels, and the development of autonomous light rail vehicles are creating a spectrum rather than a set of discrete categories. Cities are no longer asking “should we build a metro or a tram?” but “what combination of segregation, vehicle type, and automation delivers the best outcome for each corridor?” This trend is driven by two factors: the need to reduce costs (by using surface LRT where capacity allows) and the need to integrate with regional rail (via tram‑train).

However, the blurring of categories also creates challenges for standardisation. The European Union’s TSIs (Technical Specifications for Interoperability) were originally designed for mainline rail, then extended to metros, but light rail and tram‑train have often fallen between gaps. The 2024 revision of TSI LOC&PAS (Locomotives and Passenger Rolling Stock) attempts to address this by creating a new category for “urban rail vehicles” with reduced requirements for crashworthiness (since they operate at lower speeds) but additional requirements for street‑running safety (e.g., obstacle detection). The next decade will likely see a consolidation of standards around a few “urban rail” vehicle families that can be configured for any level of segregation. For infrastructure managers, this means the choice is no longer about picking a technology, but about designing a network that blends modes seamlessly – a challenge that requires not just engineering skill, but a new way of thinking about urban mobility.

— Railway News Editorial

1. Is “light rail” simply a tram that runs on a dedicated track?

Not exactly. While dedicated track is a key feature, light rail systems also typically have: (1) higher capacity vehicles (often multiple units coupled), (2) higher speeds (70‑90 km/h), (3) longer station spacing (600‑1,000 m), and (4) sometimes grade‑separated sections (tunnels or viaducts). Trams, by contrast, are designed for mixed traffic, have shorter stop spacing, and lower top speeds. The vehicle design also differs: modern LRT vehicles (e.g., Siemens S70) have higher acceleration, larger motors, and often higher floor heights (to allow for underfloor equipment). Many cities have upgraded their tram lines to LRT by adding dedicated lanes and purchasing new vehicles, but the distinction remains useful for planning purposes.

2. Why do metros use third rail power while trams use overhead wires?

Metros use third rail (typically 750 V DC) because it is less obtrusive in tunnels (no overhead wires to install) and allows a smaller tunnel cross‑section. Third rail is also more resistant to icing than overhead lines in cold climates. However, third rail is hazardous for pedestrians and cannot be used at grade crossings without extensive protection. Trams and light rail use overhead wires (catenary) because they are safer for street‑running (the wire is out of reach) and allow the use of pantographs, which are more reliable at higher speeds. Some modern LRT systems (e.g., in Bordeaux) use ground‑level power supply (APS) to eliminate overhead wires in historic city centres, but this is more expensive and complex.

3. What is the maximum capacity of a light rail line?

The theoretical maximum capacity of a light rail line is determined by train length, headway, and vehicle capacity. A modern LRV can carry 200‑300 passengers (with standees). A 3‑car train (90 m) can carry 600‑800 passengers. With a headway of 2 minutes (30 trains per hour), the capacity is 18,000‑24,000 passengers per hour per direction (pphpd). In practice, most LRT lines operate at 8,000‑15,000 pphpd. To exceed 20,000 pphpd, a line typically needs full grade separation (like a metro) or very high‑capacity vehicles (e.g., the 4‑car trains on some German Stadtbahn lines). By comparison, a metro line can handle 30,000‑80,000 pphpd.

4. Can trams and light rail vehicles share the same tracks?

Yes, but careful design is required. Trams and LRVs can share tracks if the infrastructure (track gauge, power supply, platform height) is compatible. Many systems (e.g., Karlsruhe, Berlin) have sections where trams and light rail trains run on the same tracks. However, there are technical challenges: trams typically have lower floors (300 mm) than LRVs (350‑900 mm), so platform heights must be designed to accommodate both (often a compromise with a raised kerb and retractable steps). Also, LRVs are longer and have higher acceleration, which can affect signalling and headway calculations. In mixed operation, the line is often classified as “light rail” and both vehicle types are certified for the same safety standards.

5. What is the “Karlsruhe Model” and why is it important?

The Karlsruhe Model (Germany) is a pioneering tram‑train system where tram vehicles operate on city streets and then switch to mainline railway tracks to serve suburban towns. It was first implemented in Karlsruhe in 1992 and has since been replicated in Kassel, Saarbrücken, and internationally. The model is important because it combines the accessibility of trams (frequent stops, street integration) with the reach of regional rail (high speed, longer distances). It requires dual‑voltage vehicles (750 V DC for tram lines, 15 kV AC for railway lines) and compatibility with mainline signalling (e.g., ETCS, PZB). The Karlsruhe Model has shown that tram‑trains can attract car users by providing direct, one‑seat rides from suburbs to city centres, and it has been adopted as a best practice in European urban planning.