One thing that has always intrigued me (and seems relevant here to the specification of replacement on lines which may be some way from traditional electrification), is the balance of battery technology, weight and range and charging time.
The challenge that the natural response to increase range is to add more battery capacity, but this also decreases efficiency and increases charging time.
I remember in the early days of the Nissan Leaf watching a documentary about essentially a “petrol station” which you drove into, a battery pack was then mechanically removed from the underside of the vehicle (for static charging) and a replacement fitted for a departure seconds later.
It would seem in the years since the technology has moved more towards faster charging of permanently fixed batteries, but nonetheless, on low speed lines with limited turnaround times, it seems like charging time will be a barrier to the deployment of battery technology.
Perhaps there is something in that initial technology, which would dispense with the need for “all day running” that would allow for a switchover at Inverness or Aberdeen of smaller battery units which would do a “there and back” run to Perth or on the Kyle, Far North etc.