The energy efficiency potential offered by flexible train-sets is difficult to assess in general terms. It depends on spatial and temporal demand variation and train design.

Example

The following (simplified) example may give an idea of the different influencing factors:

On a given line presently operated with 4-car train-sets, 50% of the runs have an average occupancy of 80%, the other 50% only 30%. If 2-car train-sets are introduced instead, 50% of the runs will be realised with two 2-car train-sets coupled together, the rest with only one 2-car set. The energy consumed for all the runs will be referred to as 100%.

A typical situation in suburban transport is assumed with the following components of energy consumption:

• 30% air drag
• 50% acceleration
• 20% comfort functions

Energy balance for times with high occupancy: Using two coupled 2-car sets rather than one 4-car set slightly (< 10%) increases the mass due to more traction equipment. A 10% mass increase will increase energy consumption by about 5%.

Energy balance for the rest of the day: Using a 2-car rather than a 4-car set during hours of low-demand reduces energy demand due to 3 effects:

• Energy demand for comfort functions is reduced by ~50% (=10% of the total consumption)
• With one train set instead of two and 10% higher mass per car (cf. above), mass is reduced by ~45% (=22,5 % of total energy demand)
• Due to reduced train length, air drag is reduced by ~30% (=9% of total energy demand).

Aggregating these data, we get the following energy balance through the use of shorter units:

• Runs during high-demand period consume 5% more energy.
• Runs during low-demand period consume ~41,5 % less energy
• Assuming that 50% of the energy is consumed on 50% of the runs if the same train-sets are used for all runs, one gets a total energy savings of 0,5 ? 41,5% - 0,5 ? 5% = 18%.

Conclusion

The above example simplifies the real situation. It shows however, that an energy saving potential of over 10% is realistic.