The vacuum, inital evacuation and maintenance
The energy consumption of Hyperloop is greatly reduced by running in a near-vacuum.
Energy is required to evacuate the tube initially, and maintain it, the use of steam in the tube rather than air greatly reduces the pumping energy.
Vacuum pumping energy is greatly reduced, because the steam only needs to be pumped to a low-pressure condenser
Airlock design affects the speed and energy of Hyperloop operations
Vacuum pumps are required, to evacuate the tube initially, and to pump down the airlocks. A large number of pumps are required, but the overall energy is acceptable.
The pump-down energy cost is quite low, it is paid back within the first day of operation, because Hyperloop is low energy compared to high-speed-rail.