Just for information, tram technology has progressed to the extent that sections of high floor are no longer necessary and pretty-much all new trams nowadays are fully stepless low floor (at least in the gangway, i.e. aisle and doorways, which is the accepted definition). There are some market pockets still with part high-floor (e.g. USA) but they're fading.
Fully low floor (by the same definition) diesel/gas buses have been around for more than two decades now, predominantly in Europe where they are the majority of new citybuses nowadays, but some are finding their way into Australia. This didn't have to wait for electrification. It was accomplished by the simple expedient of mounting the engine vertically across the rear of the bus so that the aisle can be stepless all the way to the rear seat row which is typically on a step-up over the engine.
The Europeans are particularly into having spaces for mobility devices near doors. In trams and buses, the entry for these is not typically the front door, but the next door along so that one can wheel into a dedicated space on the offside directly opposite the door. In Australia, with our trams being double-ended, the space is offset from the door but still adjacent. In our buses, the preference is to have the ramp at the front door, but the aisle is widened as far back as the dedicated space just to the rear of the front axle. In our trains, whether single or double deck, the dedicated space is typically adjacent to a door.
There is simply no demand to take wheeled devices further into the vehicle when there is adequate provision near a door. The impetus for having stepless gangways from front to back of public transport vehicles of any mode is simply that steps impede passenger flow and even distribution of a load, slow down passenger exchange and create a safety issue. Wheelchairs and prams aside, stepless gangways do also have a benefit for mobility-impaired passengers who are on their feet but have difficulty mounting steps or are sight-impaired.