3rd rail fails to deliver on more than one count:
It is energy inefficient to an unacceptable degree
It becomes unreliable for current collection as speeds approach the practical maximum of 100mph. Indeed, even near those speeds, maintenance needs rise sharply
As has been mentioned, ice and heavy rain causes reliability issues
High ambient temperatures can cause rail misalignment
So compared to failures of modern OLE infrastructure, it is best avoided where possible.
Thank you for your comments.
My perception, from many years use of both 3rd rail and OHLE, is that the former is less unreliable when each is operated within its normal constraints (I don't doubt that 3rd rail would be massively less reliable at, say, 125mph) and in its normal British configurations. For example, the Southern, with its plethora of electrified alternative routes, is often able to work around problems in a manner quite impossible were the wires down at, say, Shap. This may not be inherent in the technology but is in the customer experience.
I am sure that the statements you make are true as they stand, but I still enter upon an electrified trip to Scotland from Euston or King's Cross with a sense of trepidation (once I might have called it 'adventure') which is totally absent when boarding a Southern Electric. This is some mix of technical reliability, availability of mitigation and ambition of the operator - perhaps other factors too? Are you aware of any statistics which might throw light on the electrification equipment failures per train mile (3rd rail vs. OHLE) and their relative impacts?
From all that I've read efficiency seems to come down in favour of OHLE but does this relate simply to cost? If experience of OHLE is one of relatively frequent major incidents this might tend to deter travel and suggest that efficiency ought to incorporate some measure of perceived reliability.
I'm one of those awkward people who value reliability and ambiance and am less concerned with speed; perhaps I'm a little unusual in that respect but I rather doubt it.
