Currently many railway companies operate on a mixed infrastructure, i.e. trains with big speed differences (e.g. freight and high speed passenger trains) run on the same tracks. This has a number of drawbacks: low capacity, additional infrastructure costs for passing lanes etc.

Therefore infrastructure operators try to “demix” their infrastructure by restricting trains to different tracks according to average speed. Figure 1 shows (in a very simplified manner) the principle of line demixing and the consequent increase in line capacity.

This can be a way to increase system capacity without investing in new infrastructure, thus lowering specific infrastructure costs.

Presently many railways recur to temporal demixing, i.e. freight trains running during night hours, passenger trains during day-time. In many cases, the spatial demixing described here is however a much better and much more customer-friendly strategy.

Figure 1: Principle of line demixing and capacity effect (simplified)

Source: IZT

• Increased capacity
• Reduced costs
• Competition

Increased capacity

As visualized in Figure 2, demixing may considerably increase line capacity.

Studies by DB AG indicate that demixing main line passenger from freight traffic could increase line capacity by 25%. Including the local line passenger traffic in the demixing strategy may raise the capacity gain to up to 45%. These figures are however DB specific and may not be applicable as such to other railways.

Reduced costs

Increased capacity obviously reduces specific infrastructure costs. Furthermore additional infrastructure needed on mixed lines for train passing is no longer required on demixed lines. On a mixed infrastructure, all tracks have to meet the specific requirements of all services and speeds. This includes for example big curve radii and low slopes for high-speed traffic. Therefore, demixing offers additional saving potential due to reduced costs for tunnel and bridge construction.

Competition

In many mixed infrastuctures, freight traffic is restricted to night hours. In increasingly competitive transportation markets with growing customer expectations, 24 hour freight traffic is a decisive competitive factor.

Demand-side development

Since the strategy of demixing is mainly an answer to capacity problems, it is closely related to demand for train services (passenger / freight volumes).

Netz 21 programme at DB AG

Netz 21 (Network 21), an intiative for network optimisation, was drawn up by the German DB AG in the 1990s. The concept includes restructuring, standardisation, demixing and speed harmonisation, clearance of bottlenecks, the use of new technologies and a better exhaustion of transport potentials.

The eventual goal is a continuous performance network with higher customer usage and higher capacity at substantially lower cost. DB plans the step-by-step implementation to be finished by 2010.

Demixing of lines is a key element of Netz 21. The project foresees dividing up the entire network into three categories:

• a demixed “Vorrangnetz” (priority network, total track length ~10.000 km): this part of the network connects big agglomeration areas (big cities) and includes all those lines with clear speed priorities, i.e. tracks with high, medium or low speeds respectively. Railway operators may only run their trains on these lines if their trains run at the respective priority speed or if they can ensure that their trains do not disturb other trains at any time.
• a mixed “Leistungsnetz” (performance network, ~10.000 km) for mixed main-line and local-line passenger as well as freight traffic.
• a mixed “Regionalnetz” (regional network, ~17.500 km) supplements the other two networks.

DB AG estimates that the capacity of the priority and performance networks will be increased by Netz 21 by about 30%.

A positive effect of demixing on the energy efficiency of train operation is to be expected since demixing improves traffic fluidity by avoiding train stops or decelerations caused by speed differences.

There are however different effects brought about by demixing that have to be clearly distinguished:

1. Scheduled stops on the way due to slower trains letting pass faster trains are clearly reduced in a demixed infrastructure.
2. Unscheduled stops due to delayed trains impeding other trains may be reduced by demixing (cf. Figure 2).
3. Increased train density in a demixed infrastructure with raised capacity may increase number of train conflicts due to delays.

Figure 2: Schematic visualization of effect of train delys in mixed vs. demixed infrastructure (simplified)

Source: IZT

Effects 1 and 2 reduce the probability of train stops outside stations in a demixed infrastructure (as compared to the mixed case), effect 3 points in the opposite direction. It is very difficult to make a general statement on the dominance of one effect over the other and the total net effect.

Experts however tend to assume that effects 2 and 3 compensate each other yielding a zero net result. Effect 1 could then slightly improve overall “stopping statistics”.

Conclusion

Energy efficiency effects through demixing depend strongly on the individual network situation and trains densities. In most cases, the net effect will rather be a small positive one. It will however be extremely difficult to give reliable numbers in this context.