@Jane Eyre - Both
@Bete and
@disney david are correct - with an exception. Let me explain:
There are 2 kinds of bus drivers at WDW: The drivers who work for DME (Disney's Magic Express) (and the
DCL buses are included in this) and the the drivers who drive for Disney only inside the boundaries of WDW. For our purposes, we will call them DME drivers and WDW drivers.
DME drivers make runs to/from Resort Hotels, Orlando's MCO airport & the DCL using buses owned by MEARS. They are (someone correct me if I'm wrong) employed by MEARS, not WDW directly. These drivers CAN - and will - load your ECV or wheelchair or other medical equipment under the bus in the luggage hold. They also handle luggage for passengers. DME drivers will assist with loading/unloading from the external lift platform on the DME buses, however they are NOT allowed to drive or push the device onto or off of the platform; the Guest must be seated on the device and capable of driving it on/off and parking it while the platform is in motion.
WDW drivers only drive Guests within the boundaries of WDW property, using buses owned by Disney. These WDW drivers are employed directly by Disney. These drivers CANNOT load your mobility device or equipment on or off the bus, however they are allowed to provide a small amount of "assistance" with parking the device (once it is inside the bus) so that the device can be properly tied down for transit. They are also allowed to "guide" Guests when disembarking by standing at the end of the ramp outside the bus and giving direction ("turn your wheels left", etc.) As with many Cast Member positions at WDW, they are NOT allowed to accept tips from Guests. However, they LOVE to chat with Guests, and like any hard-working CM, they truly appreciate a heart-felt thank you.
If a Guest is not able to safely drive their mobility or assistive device on to or off of the bus, typically a family member or traveling companion will assist them. If no one was available, they could ask another nearby Guest to assist them, but the bus drivers (and this all applies, BTW, to Monorail CM's and boat Captains & CM's as well) are not allowed to load or unload any device, regardless of circumstance, within the internal WDW transportation system.
Guests at WDW are generally expected to be able to fully operate and manage any mobility device they use while on property, regardless of if it is a rental or personally owned.