Principle

Double-decked stock can accomodate 20-40% more seats per train length than single-decked stock. Double-decked train design is today a standard solution for many operation types including regional and suburban trains and more recently high-speed trains. Today, mature and well-experienced concepts for double-decked vehicles exist for all classes of passenger operation.

The height over rail is increased to 4600-4700 mm as opposed to less than 4000 mm for conventional stock. The weight increase has to be minimised in order to limit axle load. Therefore double-decked train design is usually combined with strong efforts to reduce weight. For instance, the TGV Duplex has aluminium bodies in contrast to steel construction in single-decked TGVs.

Examples

In suburban and regional transport, double-decked trains have become a standard option in today's railways. The examples for double-decked main line or high-speed stock are more limited:

• TGV Duplex
• Shinkansen E1.
• IC 2000

Infrastructural compatibility

The infrastructure compatibility of double-decked stock is limited but generally higher than for wide-body stock. However, many tunnels pose serious problems.

Compared to normal trains:

Since platform lengths in stations are limited (300 m in regional and 400 m in main-line stations), double-decked stock is a means to increase the capacity of a line without increasing train frequency or investing into the construction of longer platforms.

Compared to wide-body trains:

• Better compatibility with infrastructure

Passenger comfort

Seating comfort on the seats adjacent to the walls is slightly reduced due to restricted space for legs on the lower deck. This is not problematic in regional or suburban transport where average trips are short. In high-speed trains comfort issues are more relevant and could be an obstacle for double-decked stock. However, existing double-decked high-speed trains in France, Japan and elsewhere are widely accepted by passengers.

Ambulant catering

The staircases pose a problem for a mobile bistro service and for the mobility of passengers in general. In the Swiss IC 2000, a stair-free upper deck has been realized for these reasons.

Boarding times

Due to the stairs the boarding times at stations are increased with double-decked stock. This should be taken into account by operators in the timetable design. For the same reason, mixed operation of double-decked and single-decked stock should be avoided as far as possible.

Technological

• Pressure-tightness: Double-decked high-speed trains must show a high degree of pressure-tightness. Especially when entering tunnels, pressure waves must not enter the passenger coaches to avoid danger of burst ear-drums. Pressure-tight high-speed stock is still a challenge for manufacturers. Therefore, DB double-deckers may not exceed 160 km/h. TGVs are not confronted with this problem since there are no tunnels in the TGV infrastructure.
• Tilting is not possible for current double-decked stock

Infrastructure

One of the main obstacles for the introduction of double-decked stock is its incompatibility with parts of the infrastructure, especially tunnels. However, in many regional networks or in flat topography, double-decked stock can be used without any limitations.

Double-decked solutions exist for all types of passenger trains. Main limitations to its applicability are:

• incompatibility with infrastructure (tunnels etc.)
• lines requiring tilting cannot be operated with double-deckers

• TGV Duplex
• Shinkansen E1.
• IC 2000

In order to estimate the reduction of seat-specific energy consumption, single-decked and double-decked versions of an otherwise identical train (e.g. equal length etc) are compared.

Aerodynamic effect of higher car body

The cross-section of the train is increased by ~20%. Since air resistance grows with cross-sectional area in a less than proportional way, it is safe to assume that air resistance grows in the order of 10% or less.

Mass effect of higher car body

Double-decked trains are heavier than single-decked trains. The mass increment per train length is of about 10% (e.g. 11% according to a design study by Reemtsema et al. 1997).

Comfort functions

No data are available on the effect on the energy consumption of comfort functions in a double-decked train. For obvious reasons (less wall surface per seat, less interior space to be heated per seat etc), it is increased by less than the relative increase in seating capacity. 10% will be a safe upper limit here as well.

Energy consumption of the entire train

Since all components of energy consumption of a passenger train (mass, air drag and comfort energy) are increased by about 10% or less, the energy consumption will also increase by 10% or less.

Seat-specific energy demand

Since seating capacity is increased by about 20-45%, the 110% energy consumption have to be divided by 1.2 and 1.45 to get the range of seat-specific energy demand relative to a conventional car design. The result is a reduction of seat-specific energy consumption by 8% - 24%.

TGV Duplex

French TGV Duplex can be seen as a benchmark: seating capacity has been increased by 45% as compared to a single-decked train of the same length (e.g. the TVG "Réseau"). SNCF claims that this comes at virtually no increase in energy demand which means that seat-specific energy consumption is reduced by over 30%.

Fleet-wide potential

Assuming that a more than half of the regional and local lines and some of the main lines could be operated with double-decked stock in long term, there is a fleet-wide saving potential of over 5 %.

The seat-specific investment costs of double-decked stock are highly dependent on the individual features and design of the train. A 100% "ceteris paribus" comparison to single-decked stock is not possible. However, usually seat-specific costs will be lower than for conventional vehicles. In some cases this will not be true, e.g. Reemtsema, Kurz 1997 made the cost estimates on a future double-decked ICE 4 shown in the following table.

Table 1: Initial investment figures for different versions of ICE

	ICE 4	ICE 4 wide-body	ICE 4 2-decked
Investment	19.2 million EURO (100%)	22.0 million EURO (115%)	25.9 million EURO (135%)
Seats	419 (100%)	513 (122%)	506 (121%)
Specific investment per seat	45.900 EURO (100%)	42.900 EURO (93%)	51.100 EURO (111%)

Source: Reemtsema, Kurz 1997

Estimates by European Transport Consult confirm that costs are usually in favour of wide-body stock as opposed to double-decked stock.