On my recent trip, I saw a fellow carrying a desktop
computer and
LCD monitor as carry-ons - with no box.
I havre no idea waht the official restrictuions are, but I've carried the
following on British trains (on serparate occasions) :
An IBM PC/AT (got the comment from a fellow traveller 'That's some
laptop' ;-))
A DEC RS02 + 2 large bags of UNibus boards and printsets
A complete GT40
An AR88 (RCA communcations receiver)
A complete EEC rainbow with keyboard and mono monitor
A complete Torch XXX system and spares
A Philips P850
An HP9100 and HP9815
Doubtless many other medium-sized computer systems...
-tony