Hamburg's buses stay cool, but many U-Bahn and S-Bahn trains still lack air conditioning

Hamburg's public transport system presents a divided cooling reality as summer temperatures rise. Buses operated by Hochbahn, VHH, and subcontractors are all equipped with air conditioning, though the cooling strategy varies by vehicle type. Electric buses keep the interior at three degrees Celsius below the outside temperature, whereas most diesel buses are set to a constant 22 degrees. Hochbahn describes the three-degree offset as a comfort compromise, not a battery-performance limitation.

The three degrees below outside temperature are considered a compromise in passenger comfort: for some it is too cool, for others too warm.

VHH, which also runs bus services in the Hamburg region, says any bus with a defective air conditioner is immediately taken back to the depot.

Of course, such vehicles are immediately returned to the depots to be repaired.

U-Bahn: new trains cooled, old fleet cannot be retrofitted

Of Hochbahn's subway fleet, only the newer DT5 trains have air conditioning. There are 163 DT5 units, while the 126 older DT4 trains lack AC and cannot be upgraded. Hochbahn states that retrofitting would make the trains too heavy for the viaduct sections they run on. This leaves a large share of U-Bahn journeys without any active cooling.

S-Bahn: windows or six-degree cooling

Hamburg's S-Bahn, operated by Deutsche Bahn, operates a mixed fleet. The 112 older series 474 trains have no air conditioning; passengers can open the windows. The 82 newer series 490 trains are designed to lower the interior temperature by six degrees Celsius once the outside temperature reaches 27 degrees. Running the air conditioning draws about one-sixth of the vehicle's energy, according to the operator. If the cooling system fails during service, the trains continue to run, because the priority remains safe and punctual transport, a vehicle technician told the S-Bahn magazine.

Why cooling a train is hard

Frequent door opening and passenger movement make it technically difficult to maintain a stable cabin temperature. Each person radiates roughly 2.9 kilowatt-hours of heat; with 100 passengers that is 290 kilowatt-hours, enough to heat a 100-square-meter flat, notes Marco Wörner, head of vehicle technology at S-Bahn Hamburg. Trains also move through tunnels and direct sunlight, forcing the climate systems to adjust constantly between extremes.

The combination of old, non-retrofittable rolling stock, high energy consumption of air conditioning, and the everyday operational reality means that on the hottest days of the year, many Hamburg commuters will still travel without cooled air.