What may drive/necessitate a hybrid/pure EV drive-train is the CAFE standard equivalent, via EPA and NHTSA, for heavy duty trucks (e.g. class 8 vehicles).
https://www.epa.gov/newsreleases/ep...fuel-efficiency-standards-heavy-duty-trucks-0
There is a similar standard being implemented on the railroad industry here in the U.S. This is why companies like Norfolk Southern, CSX are still evaluating train related compliance (although these standards appear to be aimed at emissions, not necessarily MPG).
There are several trash companies in NYC that saw the core value of hybridizing their fleet was the savings on brake pads alone (~$5K per replacement cycle). The motor generator units and/or electrical motors create a mechanically induced load during deceleration and actually wreaked havoc on the F1 teams when the original KERS design was implemented.
A start/stop technology can save a significant amount for Disney's bus fleet in my opinion. Enabling only hotel electronics during a stop and shutting down the diesel ICE could further add to the ROI. Just think about how many buses are stopped over a given day and accrue those minutes then multiply by the number of gallons wasted.
Recently, it is my understanding that Siemens has offered to supply new generation monorail trains. It would be interesting if you negotiate with them a green/hybrid/3rd rail solution. To take it a step further, I do wonder if adding PV panels on the top of the monorails could actually be cost effective enough to aid in cooling/lighting/speakers/etc.
In regards to Prius, the NiMH battery packs (Panasonic) originally introduced actually limited the amount of energy extracted out of the cell packs (~20%). It was only after empirical evidence, coupled with laboratory testing, allowed them to open up the energy extraction to ~40% per cell pack thereby enabling them to reduce the pack size and voltage. So, it was an evolution in battery software and packaging efficiencies that was a major thrust to hybrid Prius.
My intent was not to turn this thread into technical discussions, but point out that Disney may be forced to comply to US standards that the current bus fleet needs to update/hybridize.