suplimental questions:
1) As a non technical person can some one please explain to me the limiting factors in preventing sockets (plug sockets?

) being fitted to and being able to operate on all rail vehicles.
Whilst not a technically limiting factor, I believe the age of trains, their refurbishment cycle and their expected use are the main factors in the presence or absence of power sockets today. If I can use the rolling stock typically found in Wales as an example...
The Class 175s were built around circa 2000. They have only had a minor refurbishment (new seat covers) to their interior since then. As such, they have fittings that seemed suitable twelve years ago. Most portable entertainment devices (Walkman, Game Boy, etc.) ran on replaceable batteries and portable computers were relatively few.
Pendolinos and Voyagers were built shortly thereafter for long distance travel, and with the expectation of business travellers (who were more likely to own a laptop) power sockets were fitted at tables. As
Nym pointed out, few 2002 laptops would be comfortably usable on the drop-down tables at airline seating. A mobile phone's charge would last several days, and were only useable as far as cell coverage allowed (other than playing snake or similar). These trains have not seen any refurbishment of their interiors since built (other than XC's modification of the Voyagers' shop area), so that's all that's on them today.
(As a further sign of the times, consider that all of the above were fitted with headphone sockets in the armrests of at least some areas of seating for an on-board radio service. Such a thing would be anachronistic today, but was quite modern back then.)
FGW's HST sets have been refurbished from 2005 onwards. By 2005, Ipods (non-replacable battery) have started getting popular, and laptop computers are becoming more ubiquitous. Those few power sockets provided at tables on some trains are becoming more and more hotly contested, and with FGW's refurbishment cutting the umber of tables in their high density carriages to only two, table-only coverage is not enough. So a power socket is provided for every row of airline seats.
And now on to Arriva's class 158s. These c. 1990 units had the ultra-modern provision of an on-board payphone(!) but that's long since been removed. Other than that, come 2010 they have practically the exact same interior as in the early 90s, and are due for refurbishment. People are getting used to the increasing provision of power sockets in other regional and long-distance trains, and the fact that so many people carry at least one device which frequently needs to be recharged from a wall socket means that the provision of sockets during their refurbishment is almost a foregone conclusion.
As a contrasting example, ATW's recent 153 refurbishments didn't see power sockets fitted, but these are used for local and rural services by and large.
And with regard to fitting power sockets to existing trains in the future, this is probably going to happen to all regional and long distance stock, but only as and when money is available to spend on fitting them. That's going to be more likely if the train is due for an internal refurbishment anyway.