Track Access Charge (TAC) 101
References:
https://www.sciencedirect.com/science/article/pii/S2352146525000766
Calculating a railway Track Access Charge (TAC) is a complex process that varies by country and railway system, but "normally" it is based on a set of common principles and components.
A TAC is the fee a Railway Undertaking (RU)—like a passenger or freight train company—pays to an Infrastructure Manager (IM)—the company that owns and maintains the tracks, signals, and stations (like Network Rail in the UK or DB Netz in Germany).
The primary goal of the calculation is to recover the costs of the infrastructure. Most modern charging systems, especially in Europe (based on EU directives), use a two-part tariff model.
This model breaks the charge into two main pieces:
1. Variable Charges (The "Wear and Tear" Cost)2
This is the core of the calculation and is designed to cover the direct marginal cost of your specific train running on the track. It answers the question: "What is the additional cost of maintenance and wear caused by this one train?"
This variable part is calculated using several key factors:
Distance (Train-km): The most basic factor. A 200 km journey will cost more than a 100 km journey.
Weight (Gross Tonne-km): This is critical. A heavier train (like a freight train) causes significantly more wear on the track than a light passenger train. The charge is often calculated per tonne of the train's total weight, multiplied by the distance.
Train Type & Characteristics:
Axle Load: How the weight is distributed. A high axle load causes more stress.
Speed: High-speed trains can cause different types of wear and require more advanced (and expensive) signaling.
Unsprung Mass: A technical detail about the train's suspension that affects track impact.
3
Traction (Electricity): If the train is electric, there is a separate variable charge for the electricity consumed from the overhead lines or third rail. This is often metered or modeled.
2. Fixed Charges & Markups (The "Cost of the Network")
The variable charges alone are not enough to pay for the entire railway (e.g., staff salaries, signaling centers, major bridge replacements, or interest on loans).
To cover these fixed costs, the Infrastructure Manager adds:
Fixed Access Fee: A "subscription" fee that gives the operator the right to be on the network at all.
Markups (Scarcity or Congestion Charge): This is a very common and important factor. If you want to run a train on a busy main line during the morning rush hour, you are using up scarce capacity. The IM will add a significant markup for access during this time. Running the same train at 3 AM would be much cheaper.
Market-Based Markups: Some systems add markups based on what the market can bear. For example, high-speed passenger services or specific freight (like nuclear fuel) may be charged more because they are less sensitive to price.
A Simplified Calculation Example
While the real-world formulas are extremely detailed and fill hundreds of pages in a "Network Statement" (the IM's price list), a conceptual formula might look like this:
Total TAC = (A + B + C) + D + E
A (Wear Charge):
(Cost per GTK * Gross Tonne-km)B (Scarcity Charge):
(Congestion fee for specific route/time)C (Energy Charge):
(Modeled kWh * Price per kWh)D (Infrastructure Cost Charge):
(A fixed charge for network access)E (Other Charges):
(e.g., station access fees, charges for specific services)
Who is Involved?
Infrastructure Manager (IM): Owns the tracks, calculates the charges, and publishes them in a large document (Network Statement).
Railway Undertaking (RU) / Operator: Runs the trains and pays the TAC.
Government/Regulator: An independent body (like the UK's Office of Rail and Road) that approves the IM's charging methodology to ensure it is fair, non-discriminatory, and transparent.
In summary, you calculate the track access charge by starting with the direct cost of the train's wear and tear (based on weight, distance, and type) and then adding markups to help pay for the entire fixed network, with extra fees for using busy tracks at busy times.
Track Damage Model (TDM)
Evidence for the use of the Track Damage Model (TDM)—specifically within the context of Network Rail (UK) and the broader European framework—is extensively documented in regulatory and technical reports.
The primary "evidence" for the formula you described is found in the Periodic Review documents published by the Office of Rail and Road (ORR) and Network Rail. These documents describe the transition from basic power formulas to the modern "Hybrid" Track Damage Model.
1. Primary Source: Network Rail (VTISM Analysis)
The most direct evidence for the TDM formula (which calculates the Variable Usage Charge or VUC) is the Serco VTISM Analysis Report. This report provided the engineering evidence to the ORR for the "Control Period 5" (CP5) and subsequent updates.
Document Title: VTISM Analysis to Inform the Allocation of Variable Usage Costs to Individual Vehicles
1 Key Formula Evidence: This report formally introduced the "Hybrid" formula used to calculate relative damage per axle-mile:
$$\text{Relative Damage} = 0.473 \cdot e^{0.133A} + 0.015 \cdot S \cdot U - 0.009 \cdot S - 0.284 \cdot U - 0.442$$(Where $A$ = Axle load, $S$ = Speed, $U$ = Unsprung mass)
Source Link:
Network Rail - Serco VTISM Analysis Report (PDF)
2. Regulatory Evidence: ORR Guidance
The Office of Rail and Road (ORR) provides the legal and economic justification for using these engineering models to ensure charges are "cost-reflective."
Document Title: Variable Usage Charge (VUC) User Guide
2 Evidence: The ORR guide explicitly states that the VUC is designed based on "detailed engineering models" (citing the Serco/VTISM work) to incentivize "track-friendly" rolling stock.
3 Source Link:
ORR - Variable Usage Charge Guide
3. Engineering Context: Swiss & Swedish Studies
While the formula above is the UK implementation, it cites the "Swiss/British/Swedish" studies as the baseline for damage exponents (like the $A^4$ power law for axle loads).
Swiss Context: The SBB (Swiss Federal Railways) uses a "Wear Factor" in their Train Path Pricing System.
4 Evidence of this can be found in their Network Statements.Source:
SBB Train Path Pricing System
Swedish Context: Trafikverket research (often by Andersson or Öberg) is frequently cited by Network Rail and European regulators to justify the "Marginal Cost" approach.
Evidence: See the report: Differentiated track access charges based on vehicle properties (KTH Royal Institute of Technology).
Source Link:
KTH - Differentiated Track Access Charges (PDF)
Summary of Formula Components
| Component | Evidence Source |
| UIC 714 | |
| Track Damage Model (TDM) | Serco VTISM Report (2012) for Network Rail |
| Variable Usage Charge | Network Rail CP6/CP7 Guidance Documents |
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